diff --git a/gdd.md b/gdd.md
index 3c72c77..6638cae 100644
--- a/gdd.md
+++ b/gdd.md
@@ -8,13 +8,10 @@
 * Education
 * Supply chains
 
-## Critical Path
-
-### Grid
+## Grid
 
 Add a 10x10 grid.
 Tiles are highlighted a different colour when the player hovers over them with the cursor.
-Tiles can be interacted with, which turns them another colour.
 
 ### Diffusion
 
@@ -49,7 +46,19 @@ private float TransferHeat(float t0, float alpha, float nx, float ny, float px,
 }
 ```
 
-### Tile Types
+## Build Mode
+
+Build mode can be entered by pressing the `b` key.
+Build the cursor will start flashing when entering build mode.
+
+Once in build mode, a tile type must be selected to build anything.
+Tiles cab be selected with the buttons along the bottom of the screen or by keyboard shortcuts.
+The keyboard shortcut to select a tile type is indicated on the button.
+
+To deselect a tile type, press `esc`.
+Press `esc` again to exit build mode.
+
+## Tile Types
 
 Tiles start as wild tiles.
 The player can use a UI element or a keyboard shortcut to enable build mode.
@@ -58,14 +67,14 @@ In build mode, the player can click on a wile tile to turn it into a developed t
 * Wild tiles are green
 * Developed tiles are grey
 
-### Heat
+## Heat
 
 Developed tiles produce heat while wild tiles absorb it.
 Every tile has a floating-point heat value which diffuses to its neighbours.
 Add a heat map view which can be toggled or cycled to.
 It is possible to cycle the view using a UI element.
 
-### Tile Health
+## Tile Health
 
 Add another value to wild tiles: health.
 A wild tile can accept a certain amount of heat each tick from surrounding tiles.
@@ -83,13 +92,13 @@ Its effectiveness at absorbing heat scales with its remaining health.
 
 Diffusion still occurs to move heat away from depleted wild tiles.
 
-### Rewilding
+## Rewilding
 
 Add a rewilding mode option to the UI.
 In rewilding mode, the player can click on a developed tile to turn in into a wild tile.
 The wild tile starts with 0 health.
 
-### Wild Recovery
+## Wild Recovery
 
 Depleted wile tiles can recover health each tick.
 The recovery rate is based on the difference between the damage threshold and the actual amount they received.
@@ -98,7 +107,7 @@ The recovery rate is increased if the tile has wild neighbours.
     neighbour multiplier = lerp(1, max, number of neighbours / 4)
     health restored = max((damage threshold - heat received), 0) * neighbour multiplier.
 
-### Developed tile types
+## Developed tile types
 
 Split developed tiles into 3 types:
 
@@ -111,19 +120,19 @@ These types generate different amounts of heat:
 
     industrial > commercial > residential
 
-### Journeys
+## Journeys
 
 Developed tiles have a chance to generate journeys between them.
 A journey instances a vehicle which traverses the map at a constant speed from its origin to its destination tile.
 A vehicle adds a constant amount of heat to every tile it traverses.
 
-### Moving In
+## Moving In
 
 Residential tiles have two new integer resources, current residents and maximum residents.
 Every tick there is a chance for residents to move in to a residential tile if the current number is less than the maximum number of residents.
 The initial number of maximm residents per residential tile is 4.
 
-### Jobs
+## Jobs
 
 Industrial and commercial tiles have two new integer resources, available jobs and occupied jobs.
 The chance for residents to move in depends on there being being fewer occupied than available jobs.
@@ -132,12 +141,12 @@ Their chance of generating a journey is proportional to
 
     occupied jobs / available jobs .
 
-### Commutes
+## Commutes
 
 A resident moves in to a residential tile and gets a job at an industrial or commercial 'job' tile.
 The residential tile regularly generates journeys to the job tile, and the job tile regularly generates journeys in the other direction.
 
-### Comfortable Living Temperatures
+## Comfortable Living Temperatures
 
 If a residential tile is too hot, people will not want to live there.
 Residential tiles have a critical temperature at which the chance of someone moving into a tile becomes negative, becoming a chance that someone moves out.
@@ -148,7 +157,7 @@ When someone moves out, one of the jobs associated with that residential tile is
 
 # Not critical
 
-### Wealth
+## Wealth
 
 Journeys generate a new integer tile resource, wealth.
 When a journey is completed, the destination tile accrues 1 wealth.
diff --git a/half-earth/materials/cursor.tres b/half-earth/materials/cursor.tres
index 91a714d..7cc3f5c 100644
--- a/half-earth/materials/cursor.tres
+++ b/half-earth/materials/cursor.tres
@@ -4,6 +4,6 @@
 
 [resource]
 shader = ExtResource( 1 )
-shader_param/lowColor = Color( 1, 0.960784, 0, 1 )
-shader_param/highColor = Color( 0.921569, 0, 1, 1 )
+shader_param/lowColor = Color( 0, 0, 0, 0 )
+shader_param/highColor = Color( 0, 0.976471, 1, 1 )
 shader_param/t = null
diff --git a/half-earth/nodes/debug_ui.tscn b/half-earth/nodes/debug_ui.tscn
deleted file mode 100644
index 4f2ab7f..0000000
--- a/half-earth/nodes/debug_ui.tscn
+++ /dev/null
@@ -1,36 +0,0 @@
-[gd_scene load_steps=2 format=2]
-
-[ext_resource path="res://scripts/DebugUI.cs" type="Script" id=1]
-
-[node name="Debug UI" type="Control"]
-anchor_right = 1.0
-anchor_bottom = 1.0
-script = ExtResource( 1 )
-TicksLabelPath = NodePath("Ticks")
-PauseLabelPath = NodePath("Paused")
-
-[node name="Ticks" type="Label" parent="."]
-anchor_left = 1.0
-anchor_top = 1.0
-anchor_right = 1.0
-anchor_bottom = 1.0
-margin_right = -10.0
-margin_bottom = -30.0
-grow_horizontal = 0
-grow_vertical = 0
-text = "ticks: 0"
-align = 2
-valign = 2
-
-[node name="Paused" type="Label" parent="."]
-anchor_left = 1.0
-anchor_top = 1.0
-anchor_right = 1.0
-anchor_bottom = 1.0
-margin_right = -10.0
-margin_bottom = -10.0
-grow_horizontal = 0
-grow_vertical = 0
-text = "pause: false"
-align = 2
-valign = 2
diff --git a/half-earth/nodes/game.tscn b/half-earth/nodes/game.tscn
index 9b75583..40b7b54 100644
--- a/half-earth/nodes/game.tscn
+++ b/half-earth/nodes/game.tscn
@@ -1,23 +1,59 @@
-[gd_scene load_steps=5 format=2]
+[gd_scene load_steps=11 format=2]
 
 [ext_resource path="res://nodes/grid.tscn" type="PackedScene" id=1]
 [ext_resource path="res://nodes/grid_cursor.tscn" type="PackedScene" id=2]
-[ext_resource path="res://nodes/debug_ui.tscn" type="PackedScene" id=3]
+[ext_resource path="res://nodes/ui/debug_ui.tscn" type="PackedScene" id=3]
 [ext_resource path="res://nodes/clock.tscn" type="PackedScene" id=4]
+[ext_resource path="res://nodes/interaction_modes/interaction_mode.tscn" type="PackedScene" id=5]
+[ext_resource path="res://nodes/ui/build_mode_ui.tscn" type="PackedScene" id=6]
+[ext_resource path="res://nodes/interaction_modes/build_mode.tscn" type="PackedScene" id=7]
+[ext_resource path="res://resources/tile_types/wild.tres" type="Resource" id=8]
+[ext_resource path="res://nodes/ui/mode_selection_ui.tscn" type="PackedScene" id=9]
+
+[sub_resource type="Curve" id=1]
+_data = [ Vector2( 0, 0 ), 0.0, 5.0, 0, 0, Vector2( 0.5, 1 ), 0.0, 0.0, 0, 0, Vector2( 1, 0 ), -4.0, 0.0, 0, 0 ]
 
 [node name="Root" type="Node"]
 
-[node name="Grid" parent="." instance=ExtResource( 1 )]
-DiffusionCoefficient = 0.1
-
-[node name="Cursor" parent="." instance=ExtResource( 2 )]
-Grid = NodePath("../Grid")
-
-[node name="Debug UI" parent="." instance=ExtResource( 3 )]
-
 [node name="Clock" parent="." instance=ExtResource( 4 )]
 _autoTick = true
 
-[connection signal="OnPauseChanged" from="Clock" to="Debug UI" method="_on_Clock_OnPauseChanged"]
+[node name="Grid" parent="." instance=ExtResource( 1 )]
+DiffusionCoefficient = 0.1
+_startTileTypeResource = ExtResource( 8 )
+
+[node name="Cursor" parent="." instance=ExtResource( 2 )]
+Grid = NodePath("../Grid")
+_pulseShape = SubResource( 1 )
+
+[node name="Interaction Modes" parent="." instance=ExtResource( 5 )]
+_buildModePath = NodePath("Build Mode")
+
+[node name="Build Mode" parent="Interaction Modes" instance=ExtResource( 7 )]
+_gridPath = NodePath("../../Grid")
+_cursorPath = NodePath("../../Cursor")
+
+[node name="UI" type="Control" parent="."]
+anchor_right = 1.0
+anchor_bottom = 1.0
+mouse_filter = 2
+
+[node name="Debug" parent="UI" instance=ExtResource( 3 )]
+
+[node name="Build Mode" parent="UI" instance=ExtResource( 6 )]
+_buildModePath = NodePath("../../Interaction Modes/Build Mode")
+
+[node name="Mode Selection" parent="UI" instance=ExtResource( 9 )]
+
+[connection signal="OnPauseChanged" from="Clock" to="UI/Debug" method="_on_Clock_OnPauseChanged"]
 [connection signal="OnTick" from="Clock" to="Grid" method="_on_Clock_OnTick"]
-[connection signal="OnTick" from="Clock" to="Debug UI" method="_on_Clock_OnTick"]
+[connection signal="OnTick" from="Clock" to="UI/Debug" method="_on_Clock_OnTick"]
+[connection signal="OnInteractionModeChanged" from="Interaction Modes" to="Cursor" method="_on_Interaction_Mode_OnInteractionModeChanged"]
+[connection signal="OnInteractionModeChanged" from="Interaction Modes" to="UI/Debug" method="_on_Interaction_Mode_OnInteractionModeChanged"]
+[connection signal="OnModeEntered" from="Interaction Modes/Build Mode" to="Interaction Modes" method="_on_Build_Mode_OnModeEntered"]
+[connection signal="OnModeEntered" from="Interaction Modes/Build Mode" to="UI/Mode Selection" method="EnableBuildMode"]
+[connection signal="OnModeExited" from="Interaction Modes/Build Mode" to="Interaction Modes" method="_on_Build_Mode_OnModeExited"]
+[connection signal="SelectedTileTypeChanged" from="Interaction Modes/Build Mode" to="UI/Build Mode" method="UpdateButtonToggleState"]
+[connection signal="OnExit" from="UI/Build Mode" to="Interaction Modes" method="Reset"]
+[connection signal="OnExit" from="UI/Build Mode" to="UI/Mode Selection" method="Enable"]
+[connection signal="BuildModeEnabled" from="UI/Mode Selection" to="Interaction Modes/Build Mode" method="Enable"]
diff --git a/half-earth/nodes/interaction_modes/build_mode.tscn b/half-earth/nodes/interaction_modes/build_mode.tscn
new file mode 100644
index 0000000..4c2ca0e
--- /dev/null
+++ b/half-earth/nodes/interaction_modes/build_mode.tscn
@@ -0,0 +1,9 @@
+[gd_scene load_steps=3 format=2]
+
+[ext_resource path="res://scripts/interaction_modes/BuildMode.cs" type="Script" id=1]
+[ext_resource path="res://resources/tile_types/tile_type_collections/buildable_tiles.tres" type="Resource" id=2]
+
+[node name="Build Mode" type="Node"]
+script = ExtResource( 1 )
+_key = 66
+_buildableTilesResource = ExtResource( 2 )
diff --git a/half-earth/nodes/interaction_modes/interaction_mode.tscn b/half-earth/nodes/interaction_modes/interaction_mode.tscn
new file mode 100644
index 0000000..c5caa48
--- /dev/null
+++ b/half-earth/nodes/interaction_modes/interaction_mode.tscn
@@ -0,0 +1,6 @@
+[gd_scene load_steps=2 format=2]
+
+[ext_resource path="res://scripts/interaction_modes/InteractionModes.cs" type="Script" id=1]
+
+[node name="Interaction Mode" type="Node"]
+script = ExtResource( 1 )
diff --git a/half-earth/nodes/interaction_modes/selection_mode.tscn b/half-earth/nodes/interaction_modes/selection_mode.tscn
new file mode 100644
index 0000000..4e80ec9
--- /dev/null
+++ b/half-earth/nodes/interaction_modes/selection_mode.tscn
@@ -0,0 +1,3 @@
+[gd_scene format=2]
+
+[node name="Selection Mode" type="Node"]
diff --git a/half-earth/nodes/ui/build_mode_ui.tscn b/half-earth/nodes/ui/build_mode_ui.tscn
new file mode 100644
index 0000000..2350c06
--- /dev/null
+++ b/half-earth/nodes/ui/build_mode_ui.tscn
@@ -0,0 +1,18 @@
+[gd_scene load_steps=2 format=2]
+
+[ext_resource path="res://scripts/ui/BuildModeUI.cs" type="Script" id=1]
+
+[node name="Build Mode" type="Control"]
+anchor_right = 1.0
+anchor_bottom = 1.0
+script = ExtResource( 1 )
+
+[node name="Exit" type="Button" parent="."]
+anchor_top = 1.0
+anchor_bottom = 1.0
+margin_top = -40.0
+margin_right = 40.0
+grow_vertical = 0
+text = "Exit"
+
+[connection signal="pressed" from="Exit" to="." method="Exit"]
diff --git a/half-earth/nodes/ui/debug_ui.tscn b/half-earth/nodes/ui/debug_ui.tscn
new file mode 100644
index 0000000..6542174
--- /dev/null
+++ b/half-earth/nodes/ui/debug_ui.tscn
@@ -0,0 +1,48 @@
+[gd_scene load_steps=2 format=2]
+
+[ext_resource path="res://scripts/ui/DebugUI.cs" type="Script" id=1]
+
+[node name="Debug UI" type="Control"]
+anchor_left = 1.0
+anchor_right = 1.0
+mouse_filter = 2
+script = ExtResource( 1 )
+TicksLabelPath = NodePath("Ticks")
+PauseLabelPath = NodePath("Paused")
+InteractionModeLabelPath = NodePath("Interaction Mode")
+
+[node name="Interaction Mode" type="Label" parent="."]
+anchor_left = 1.0
+anchor_right = 1.0
+margin_left = -102.0
+margin_top = 15.0
+margin_right = -14.0
+margin_bottom = 29.0
+grow_horizontal = 0
+grow_vertical = 0
+text = "mode: Selection"
+align = 2
+
+[node name="Ticks" type="Label" parent="."]
+anchor_left = 1.0
+anchor_right = 1.0
+margin_left = -60.0
+margin_top = 35.0
+margin_right = -14.0
+margin_bottom = 49.0
+grow_horizontal = 0
+grow_vertical = 0
+text = "ticks: 0"
+align = 2
+
+[node name="Paused" type="Label" parent="."]
+anchor_left = 1.0
+anchor_right = 1.0
+margin_left = -90.0
+margin_top = 55.0
+margin_right = -14.0
+margin_bottom = 69.0
+grow_horizontal = 0
+grow_vertical = 0
+text = "pause: false"
+align = 2
diff --git a/half-earth/nodes/ui/mode_selection_ui.tscn b/half-earth/nodes/ui/mode_selection_ui.tscn
new file mode 100644
index 0000000..1916be8
--- /dev/null
+++ b/half-earth/nodes/ui/mode_selection_ui.tscn
@@ -0,0 +1,19 @@
+[gd_scene load_steps=2 format=2]
+
+[ext_resource path="res://scripts/ui/ModeSelectionUI.cs" type="Script" id=1]
+
+[node name="Mode Selection UI" type="Control"]
+anchor_top = 1.0
+anchor_bottom = 1.0
+margin_top = -600.0
+margin_bottom = -600.0
+script = ExtResource( 1 )
+
+[node name="Build Mode" type="Button" parent="."]
+margin_left = 2.0
+margin_top = 562.0
+margin_right = 38.0
+margin_bottom = 598.0
+text = "Build Mode"
+
+[connection signal="pressed" from="Build Mode" to="." method="EnableBuildMode"]
diff --git a/half-earth/resources/tile_types/developed.tres b/half-earth/resources/tile_types/developed.tres
new file mode 100644
index 0000000..011fa7b
--- /dev/null
+++ b/half-earth/resources/tile_types/developed.tres
@@ -0,0 +1,10 @@
+[gd_resource type="Resource" load_steps=2 format=2]
+
+[ext_resource path="res://scripts/TileType.cs" type="Script" id=1]
+
+[resource]
+script = ExtResource( 1 )
+Name = "Developed"
+BuildLabel = "[D]eveloped"
+Key = 68
+Color = Color( 0.372549, 0.372549, 0.372549, 1 )
diff --git a/half-earth/resources/tile_types/tile_type_collections/buildable_tiles.tres b/half-earth/resources/tile_types/tile_type_collections/buildable_tiles.tres
new file mode 100644
index 0000000..0491e19
--- /dev/null
+++ b/half-earth/resources/tile_types/tile_type_collections/buildable_tiles.tres
@@ -0,0 +1,8 @@
+[gd_resource type="Resource" load_steps=3 format=2]
+
+[ext_resource path="res://scripts/TileTypeCollection.cs" type="Script" id=1]
+[ext_resource path="res://resources/tile_types/developed.tres" type="Resource" id=2]
+
+[resource]
+script = ExtResource( 1 )
+_tileTypeResources = [ ExtResource( 2 ) ]
diff --git a/half-earth/resources/tile_types/wild.tres b/half-earth/resources/tile_types/wild.tres
new file mode 100644
index 0000000..16ebc36
--- /dev/null
+++ b/half-earth/resources/tile_types/wild.tres
@@ -0,0 +1,10 @@
+[gd_resource type="Resource" load_steps=2 format=2]
+
+[ext_resource path="res://scripts/TileType.cs" type="Script" id=1]
+
+[resource]
+script = ExtResource( 1 )
+Name = "Wild"
+BuildLabel = "[W]ild"
+Key = 87
+Color = Color( 0, 0.545098, 0.0196078, 1 )
diff --git a/half-earth/scripts/DebugUI.cs b/half-earth/scripts/DebugUI.cs
deleted file mode 100644
index 8adc836..0000000
--- a/half-earth/scripts/DebugUI.cs
+++ /dev/null
@@ -1,37 +0,0 @@
-using Godot;
-using System;
-
-public class DebugUI : Control
-{
-    [Export]
-    public NodePath TicksLabelPath { private get; set; }
-    private Label _ticksLabel;
-
-    [Export]
-    public NodePath PauseLabelPath { private get; set; }
-    private Label _pauseLabel;
-
-    public override void _Ready()
-    {
-        _ticksLabel = GetNode<Label>(TicksLabelPath);
-        _pauseLabel = GetNode<Label>(PauseLabelPath);
-
-        SetTicksText(0);
-        SetPauseText(false);
-    }
-
-    #region Signals
-    public void _on_Clock_OnPauseChanged(bool paused) => SetPauseText(paused);
-    public void _on_Clock_OnTick(int ticks) => SetTicksText(ticks);
-    #endregion
-
-    private void SetTicksText(int ticks)
-    {
-        _ticksLabel.Text = $"ticks: {ticks}";
-    }
-
-    private void SetPauseText(bool paused)
-    {
-        _pauseLabel.Text = $"pause: {paused}";
-    }
-}
diff --git a/half-earth/scripts/GridCursor.cs b/half-earth/scripts/GridCursor.cs
index c2f1631..b41a5fd 100644
--- a/half-earth/scripts/GridCursor.cs
+++ b/half-earth/scripts/GridCursor.cs
@@ -8,8 +8,22 @@ public class GridCursor : Sprite
     private WorldGrid _grid;
     private ShaderMaterial _material;
 
+    #region Animation
+    [Export]
+    private Curve _pulseShape;
+    [Export]
+    private float _pulseRate = 1.0f;
+    private float PulsePeriod => 1.0f / _pulseRate;
+    #endregion
+
+    private float _elapsed = 0.0f;
+    private bool _pulsing = false;
+
     private const string T = "t";
 
+    public int X { get; private set; }
+    public int Y { get; private set; }
+
     public override void _Ready()
     {
         _grid = GetNode<WorldGrid>(Grid);
@@ -24,28 +38,32 @@ public class GridCursor : Sprite
         {
             var pos = mouseMoveEvent.Position;
             _grid.GetGridPos(pos, out var x, out var y);
-            this.Visible = _grid.IsInBounds(x, y);
-            var position = new Vector2(x + .5f, y + .5f) * _grid.CellSize;
+            X = x;
+            Y = y;
+
+            this.Visible = _grid.IsInBounds(X, Y);
+            var position = new Vector2(X + .5f, Y + .5f) * _grid.CellSize;
             this.Position = position;
-        }
-        else if (@event is InputEventMouseButton mouseButtonEvent)
-        {
-            switch ((ButtonList)mouseButtonEvent.ButtonIndex)
-            {
-                case ButtonList.Left:
-                    if (mouseButtonEvent.Pressed)
-                    {
-                        var pos = mouseButtonEvent.Position;
-                        _grid.GetGridPos(pos, out var x, out var y);
-                        _grid.SetTileValue(x, y, 1.0f);
-                        _material.SetShaderParam(T, 1f);
-                    }
-                    else
-                    {
-                        _material.SetShaderParam(T, 0f);
-                    }
-                    break;
-            }
+            return;
         }
     }
+
+    public override void _Process(float delta)
+    {
+        _elapsed += delta * _pulseRate;
+
+        if (!_pulsing)
+        {
+            _material.SetShaderParam(T, 1f);
+            return;
+        }
+
+        var a = _pulseShape.Interpolate(_elapsed % 1.0f);
+        _material.SetShaderParam(T, a);
+    }
+
+    public void _on_Interaction_Mode_OnInteractionModeChanged(Mode oldMode, Mode newMode)
+    {
+        _pulsing = newMode is BuildMode;
+    }
 }
diff --git a/half-earth/scripts/Tile.cs b/half-earth/scripts/Tile.cs
index f46e5ee..70560a0 100644
--- a/half-earth/scripts/Tile.cs
+++ b/half-earth/scripts/Tile.cs
@@ -5,4 +5,5 @@ public struct Tile
     public bool isHighlighted;
     public float value;
     public ShaderMaterial material;
+    public TileType type;
 }
\ No newline at end of file
diff --git a/half-earth/scripts/TileType.cs b/half-earth/scripts/TileType.cs
new file mode 100644
index 0000000..c6dc2ab
--- /dev/null
+++ b/half-earth/scripts/TileType.cs
@@ -0,0 +1,21 @@
+using Godot;
+
+public class TileType : Resource
+{
+    [Export]
+    public string Name { get; private set; }
+
+    [Export]
+    public string BuildLabel { get; private set; }
+
+    [Export]
+    public KeyList Key { get; private set; }
+
+    [Export]
+    public Color Color { get; private set; }
+
+    public override string ToString()
+    {
+        return Name;
+    }
+}
\ No newline at end of file
diff --git a/half-earth/scripts/TileTypeCollection.cs b/half-earth/scripts/TileTypeCollection.cs
new file mode 100644
index 0000000..d30c6e8
--- /dev/null
+++ b/half-earth/scripts/TileTypeCollection.cs
@@ -0,0 +1,46 @@
+using Godot;
+using System;
+using System.Collections;
+using System.Collections.Generic;
+
+public class TileTypeCollection : Resource, IReadOnlyList<TileType>
+{
+    [Export]
+    private List<Resource> _tileTypeResources = new List<Resource>();
+
+    private List<TileType> TileTypes
+    {
+        get
+        {
+            if (_tileTypes != null)
+                return _tileTypes;
+
+            _tileTypes = new List<TileType>();
+
+            foreach (var resource in _tileTypeResources)
+            {
+                if (!(resource is TileType))
+                    throw new InvalidCastException($"{resource} must be a {typeof(TileType)}");
+
+                _tileTypes.Add((TileType)resource);
+            }
+
+            return _tileTypes;
+        }
+    }
+    private List<TileType> _tileTypes = null;
+
+    public TileType this[int index] => TileTypes[index];
+
+    public int Count => TileTypes.Count;
+
+    public IEnumerator<TileType> GetEnumerator()
+    {
+        return TileTypes.GetEnumerator();
+    }
+
+    IEnumerator IEnumerable.GetEnumerator()
+    {
+        return TileTypes.GetEnumerator();
+    }
+}
diff --git a/half-earth/scripts/WorldGrid.cs b/half-earth/scripts/WorldGrid.cs
index 45e7e2b..661b202 100644
--- a/half-earth/scripts/WorldGrid.cs
+++ b/half-earth/scripts/WorldGrid.cs
@@ -6,6 +6,10 @@ public class WorldGrid : Node2D
     [Export]
     private PackedScene TileScene { get; set; }
 
+    [Export]
+    private Resource _startTileTypeResource;
+    private TileType _startTileType;
+
     [Export]
     public int Size { get; set; }
 
@@ -21,6 +25,8 @@ public class WorldGrid : Node2D
     // Called when the node enters the scene tree for the first time.
     public override void _Ready()
     {
+        _startTileType = (TileType)_startTileTypeResource;
+
         GenerateGrid(Size);
     }
 
@@ -52,9 +58,14 @@ public class WorldGrid : Node2D
         _tiles[x, y].isHighlighted ^= true;
     }
 
-    public void SetTileValue(int x, int y, float value)
+    public void SetTileType(int x, int y, TileType tileType)
     {
-        _tiles[x, y].value = value;
+        if (!IsInBounds(x, y))
+            return;
+
+        GD.Print($"set ({x}, {y}) to {tileType}");
+        _tiles[x, y].type = tileType;
+        _tiles[x, y].material.SetShaderParam("lowColor", tileType.Color);
     }
 
     public void GetGridPos(Vector2 position, out int x, out int y)
@@ -90,6 +101,7 @@ public class WorldGrid : Node2D
                 var tile = new Tile();
                 tile.isHighlighted = false;
                 tile.value = 0.0f;
+                tile.type = _startTileType;
 
                 var node = TileScene.Instance<Node2D>();
                 this.AddChild(node);
@@ -101,6 +113,7 @@ public class WorldGrid : Node2D
                 node.Position = position;
                 var canvasItem = (CanvasItem)node;
                 tile.material = (ShaderMaterial)canvasItem.Material;
+                tile.material.SetShaderParam("lowColor", _startTileType.Color);
 
                 _tiles[x, y] = tile;
             }
diff --git a/half-earth/scripts/interaction_modes/BuildMode.cs b/half-earth/scripts/interaction_modes/BuildMode.cs
new file mode 100644
index 0000000..d2b2b25
--- /dev/null
+++ b/half-earth/scripts/interaction_modes/BuildMode.cs
@@ -0,0 +1,132 @@
+using Godot;
+using System;
+
+public class BuildMode : Mode
+{
+    [Signal]
+    delegate void SelectedTileTypeChanged();
+
+    [Export]
+    private NodePath _gridPath;
+    private WorldGrid _grid;
+
+    [Export]
+    private NodePath _cursorPath;
+    private GridCursor _cursor;
+
+    [Export]
+    private Resource _buildableTilesResource;
+    public TileTypeCollection BuildableTiles { get; private set; }
+
+    public TileType SelectedTileType
+    {
+        get => _selectedTileType;
+        set
+        {
+            if (_selectedTileType == value)
+                return;
+
+            if (value != null)
+            {
+                GD.Print($"{GetType()}: selected {value} tile type");
+            }
+            else if (_selectedTileType != null)
+            {
+                GD.Print($"{GetType()}: cleared tile selection");
+            }
+
+            _selectedTileType = value;
+            EmitSignal(nameof(SelectedTileTypeChanged), _selectedTileType);
+        }
+    }
+    private TileType _selectedTileType = null;
+
+    public override void _Ready()
+    {
+        base._Ready();
+
+        BuildableTiles = (TileTypeCollection)_buildableTilesResource;
+
+        foreach (var tt in BuildableTiles)
+        {
+            GD.Print(tt.Name);
+        }
+
+        _grid = GetNode<WorldGrid>(_gridPath);
+        _cursor = GetNode<GridCursor>(_cursorPath);
+    }
+
+    private void HandleKeyInput(InputEventKey keyEvent)
+    {
+        if (!(keyEvent.Pressed))
+            return;
+
+        var key = (KeyList)keyEvent.Scancode;
+
+        if (key == KeyList.Escape)
+        {
+            if (SelectedTileType != null)
+            {
+                SelectedTileType = null;
+                return;
+            }
+
+            Disable();
+        }
+
+        foreach (var tt in BuildableTiles)
+        {
+            if (key != tt.Key)
+                continue;
+
+            SelectedTileType = tt;
+        }
+    }
+
+    private void HandleMouseInput(InputEventMouse mouseEvent)
+    {
+        if (!(mouseEvent is InputEventMouseButton buttonEvent))
+            return;
+
+        if (!buttonEvent.Pressed)
+            return;
+
+        var button = (ButtonList)buttonEvent.ButtonIndex;
+        if (button != ButtonList.Left)
+            return;
+
+        if (SelectedTileType == null)
+            return;
+
+        _grid.SetTileType(_cursor.X, _cursor.Y, SelectedTileType);
+    }
+
+    public override void _Input(InputEvent @event)
+    {
+        base._Input(@event);
+
+        if (!Active)
+            return;
+
+        if (@event is InputEventKey keyEvent)
+        {
+            HandleKeyInput(keyEvent);
+            return;
+        }
+
+        if (@event is InputEventMouse mouseEvent)
+        {
+            HandleMouseInput(mouseEvent);
+            return;
+        }
+    }
+
+    protected override void OnEnabled()
+    {
+    }
+
+    protected override void OnDisabled()
+    {
+        SelectedTileType = null;
+    }
+}
diff --git a/half-earth/scripts/interaction_modes/InteractionModes.cs b/half-earth/scripts/interaction_modes/InteractionModes.cs
new file mode 100644
index 0000000..9536ff0
--- /dev/null
+++ b/half-earth/scripts/interaction_modes/InteractionModes.cs
@@ -0,0 +1,57 @@
+using Godot;
+using System;
+
+public class InteractionModes : Node
+{
+    [Export]
+    private NodePath _buildModePath;
+    private BuildMode _buildMode;
+
+    [Signal]
+    delegate void OnInteractionModeChanged(Mode oldMode, Mode newMode);
+
+    private Mode Mode
+    {
+        get => _current;
+        set
+        {
+            if (_current == value)
+                return;
+
+            if (value != null)
+            {
+                GD.Print($"set interaction mode: {value}");
+            }
+            else if (_current != null)
+            {
+                GD.Print($"clear interaction mode");
+            }
+
+            var old = _current;
+            _current = value;
+            EmitSignal(nameof(OnInteractionModeChanged), old, _current);
+        }
+    }
+    private Mode _current = null;
+
+    public override void _Ready()
+    {
+        base._Ready();
+
+        _buildMode = GetNode<BuildMode>(_buildModePath);
+    }
+
+    public void _on_Build_Mode_OnModeEntered()
+    {
+        Mode = _buildMode;
+    }
+    public void _on_Build_Mode_OnModeExited()
+    {
+        Mode = null;
+    }
+
+    public void Reset()
+    {
+        _buildMode.Disable();
+    }
+}
diff --git a/half-earth/scripts/interaction_modes/Mode.cs b/half-earth/scripts/interaction_modes/Mode.cs
new file mode 100644
index 0000000..59e8ade
--- /dev/null
+++ b/half-earth/scripts/interaction_modes/Mode.cs
@@ -0,0 +1,68 @@
+using Godot;
+using System;
+
+public abstract class Mode : Node
+{
+    [Signal]
+    delegate void OnModeEntered();
+    [Signal]
+    delegate void OnModeExited();
+
+    [Export]
+    private KeyList _key;
+
+    public bool Active
+    {
+        get => _active;
+        private set
+        {
+            if (value == _active)
+                return;
+
+            _active = value;
+
+            var signal = _active
+                ? nameof(OnModeEntered)
+                : nameof(OnModeExited);
+            EmitSignal(signal);
+        }
+    }
+    private bool _active;
+
+    public override void _Input(InputEvent @event)
+    {
+        base._Input(@event);
+
+        if (!(@event is InputEventKey keyEvent))
+            return;
+
+        if (!keyEvent.Pressed)
+            return;
+
+        var key = (KeyList)keyEvent.Scancode;
+
+        if (key == _key)
+        {
+            Active = true;
+        }
+    }
+
+    public void Enable()
+    {
+        Active = true;
+        OnEnabled();
+    }
+    protected abstract void OnEnabled();
+
+    public void Disable()
+    {
+        Active = false;
+        OnDisabled();
+    }
+    protected abstract void OnDisabled();
+
+    public override string ToString()
+    {
+        return GetType().ToString();
+    }
+}
diff --git a/half-earth/scripts/ui/BuildModeUI.cs b/half-earth/scripts/ui/BuildModeUI.cs
new file mode 100644
index 0000000..92ddfa0
--- /dev/null
+++ b/half-earth/scripts/ui/BuildModeUI.cs
@@ -0,0 +1,81 @@
+using Godot;
+using System;
+using System.Collections.Generic;
+
+public class BuildModeUI : Control
+{
+    [Signal]
+    delegate void OnExit();
+
+    [Export]
+    private NodePath _buildModePath;
+    private BuildMode _buildMode;
+
+    private readonly Dictionary<TileType, Button> _typeButtons
+        = new Dictionary<TileType, Button>();
+
+    // Called when the node enters the scene tree for the first time.
+    public override void _Ready()
+    {
+        _buildMode = GetNode<BuildMode>(_buildModePath);
+
+        var buttonHeight = 40;
+        var tileType = _buildMode.BuildableTiles[0];
+        int x = 50;
+
+        SpawnButton(tileType, buttonHeight, ref x);
+    }
+
+    private void UpdateButtonToggleState(TileType tileType)
+    {
+        foreach (var kvp in _typeButtons)
+        {
+            var button = _typeButtons[kvp.Key];
+            button.SetPressedNoSignal(kvp.Key == tileType);
+        }
+    }
+
+    private void SpawnButton(TileType tileType, int buttonHeight, ref int x)
+    {
+        const int MARGIN = 5;
+
+        var button = new Button();
+        this.AddChild(button);
+
+        button.Text = tileType.BuildLabel;
+        var parameters = new Godot.Collections.Array(button, tileType);
+        button.Connect("pressed", this, nameof(SelectTileType), parameters);
+
+        button.SetAnchorsPreset(LayoutPreset.BottomLeft);
+        button.MarginBottom = 0;
+
+        var size = button.RectSize;
+        button.RectSize = new Vector2(size.x, buttonHeight);
+
+        button.ToggleMode = true;
+
+        button.SetPosition(new Vector2(x, -buttonHeight));
+        x += Mathf.RoundToInt(size.x) + MARGIN;
+
+        _typeButtons[tileType] = button;
+    }
+
+    private void SelectTileType(Button button, TileType tileType)
+    {
+        _buildMode.SelectedTileType = button.Pressed
+            ? tileType
+            : null;
+    }
+
+    public void Exit()
+    {
+        EmitSignal(nameof(OnExit));
+    }
+
+    public override void _Process(float delta)
+    {
+        base._Process(delta);
+
+        Visible = _buildMode.Active;
+    }
+}
diff --git a/half-earth/scripts/ui/DebugUI.cs b/half-earth/scripts/ui/DebugUI.cs
new file mode 100644
index 0000000..c810527
--- /dev/null
+++ b/half-earth/scripts/ui/DebugUI.cs
@@ -0,0 +1,41 @@
+using Godot;
+using System;
+
+public class DebugUI : Control
+{
+    [Export]
+    public NodePath TicksLabelPath { private get; set; }
+    private Label _ticksLabel;
+
+    [Export]
+    public NodePath PauseLabelPath { private get; set; }
+    private Label _pauseLabel;
+
+    [Export]
+    public NodePath InteractionModeLabelPath { private get; set; }
+    private Label _interactionModeLabel;
+    public override void _Ready()
+    {
+        _ticksLabel = GetNode<Label>(TicksLabelPath);
+        _pauseLabel = GetNode<Label>(PauseLabelPath);
+        _interactionModeLabel = GetNode<Label>(InteractionModeLabelPath);
+    }
+
+    #region Signals
+    public void _on_Clock_OnPauseChanged(bool paused) =>
+        SetLabelText(_pauseLabel, "pause", paused);
+
+    public void _on_Clock_OnTick(int ticks) =>
+        SetLabelText(_ticksLabel, "ticks", ticks);
+
+    public void _on_Interaction_Mode_OnInteractionModeChanged(Mode oldMode, Mode newMode)
+    {
+        SetLabelText(_interactionModeLabel, "mode", newMode);
+    }
+    #endregion
+
+    private void SetLabelText(Label label, string key, object value)
+    {
+        label.Text = $"{key}: {value}";
+    }
+}
diff --git a/half-earth/scripts/ui/ModeSelectionUI.cs b/half-earth/scripts/ui/ModeSelectionUI.cs
new file mode 100644
index 0000000..5c8a4bd
--- /dev/null
+++ b/half-earth/scripts/ui/ModeSelectionUI.cs
@@ -0,0 +1,24 @@
+using Godot;
+using System;
+
+public class ModeSelectionUI : Control
+{
+    [Signal]
+    delegate void BuildModeEnabled();
+
+    public override void _Process(float delta)
+    {
+        base._Process(delta);
+    }
+
+    public void EnableBuildMode()
+    {
+        EmitSignal(nameof(BuildModeEnabled));
+        Visible = false;
+    }
+
+    public void Enable()
+    {
+        Visible = true;
+    }
+}