diff --git a/half-earth/nodes/game.tscn b/half-earth/nodes/game.tscn
index c86e8f4..bde9c19 100644
--- a/half-earth/nodes/game.tscn
+++ b/half-earth/nodes/game.tscn
@@ -1,4 +1,4 @@
-[gd_scene load_steps=11 format=2]
+[gd_scene load_steps=12 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]
@@ -9,6 +9,7 @@
 [ext_resource path="res://nodes/interaction_modes/build_mode.tscn" type="PackedScene" id=7]
 [ext_resource path="res://resources/tiles/tiles.tres" type="Resource" id=8]
 [ext_resource path="res://nodes/ui/mode_selection_ui.tscn" type="PackedScene" id=9]
+[ext_resource path="res://nodes/overlays.tscn" type="PackedScene" id=10]
 
 [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 ]
@@ -18,21 +19,24 @@ _data = [ Vector2( 0, 0 ), 0.0, 5.0, 0, 0, Vector2( 0.5, 1 ), 0.0, 0.0, 0, 0, Ve
 [node name="Clock" parent="." instance=ExtResource( 4 )]
 _autoTick = true
 
-[node name="Grid" parent="." instance=ExtResource( 1 )]
+[node name="World Grid" parent="." instance=ExtResource( 1 )]
 DiffusionCoefficient = 0.1
 _tileGridResource = ExtResource( 8 )
 
 [node name="Cursor" parent="." instance=ExtResource( 2 )]
-Grid = NodePath("../Grid")
+Grid = NodePath("../World 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")
+_gridPath = NodePath("../../World Grid")
 _cursorPath = NodePath("../../Cursor")
 
+[node name="Overlays" parent="." instance=ExtResource( 10 )]
+_worldGridPath = NodePath("../World Grid")
+
 [node name="UI" type="Control" parent="."]
 anchor_right = 1.0
 anchor_bottom = 1.0
@@ -46,7 +50,7 @@ _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="World Grid" 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"]
diff --git a/half-earth/nodes/overlays.tscn b/half-earth/nodes/overlays.tscn
new file mode 100644
index 0000000..1e6405b
--- /dev/null
+++ b/half-earth/nodes/overlays.tscn
@@ -0,0 +1,12 @@
+[gd_scene load_steps=5 format=2]
+
+[ext_resource path="res://scripts/overlays/Overlays.cs" type="Script" id=1]
+[ext_resource path="res://resources/overlays/heat_overlay.tres" type="Resource" id=2]
+[ext_resource path="res://resources/overlays/no_overlay.tres" type="Resource" id=3]
+[ext_resource path="res://resources/tiles/tiles.tres" type="Resource" id=4]
+
+[node name="Overlays" type="Node"]
+script = ExtResource( 1 )
+_cycleOverlayKey = 16777218
+_overlayResources = [ ExtResource( 3 ), ExtResource( 2 ) ]
+_tileGridResource = ExtResource( 4 )
diff --git a/half-earth/resources/overlays/heat_overlay.tres b/half-earth/resources/overlays/heat_overlay.tres
new file mode 100644
index 0000000..5f1c269
--- /dev/null
+++ b/half-earth/resources/overlays/heat_overlay.tres
@@ -0,0 +1,8 @@
+[gd_resource type="Resource" load_steps=2 format=2]
+
+[ext_resource path="res://scripts/overlays/HeatOverlay.cs" type="Script" id=1]
+
+[resource]
+script = ExtResource( 1 )
+_coldColor = Color( 0, 0, 0, 1 )
+_hotColor = Color( 1, 0.654902, 0, 1 )
diff --git a/half-earth/resources/overlays/no_overlay.tres b/half-earth/resources/overlays/no_overlay.tres
new file mode 100644
index 0000000..d4a783f
--- /dev/null
+++ b/half-earth/resources/overlays/no_overlay.tres
@@ -0,0 +1,6 @@
+[gd_resource type="Resource" load_steps=2 format=2]
+
+[ext_resource path="res://scripts/overlays/NoOverlay.cs" type="Script" id=1]
+
+[resource]
+script = ExtResource( 1 )
diff --git a/half-earth/resources/tiles/tile_types/developed.tres b/half-earth/resources/tiles/tile_types/developed.tres
index 1efdccf..70a9358 100644
--- a/half-earth/resources/tiles/tile_types/developed.tres
+++ b/half-earth/resources/tiles/tile_types/developed.tres
@@ -8,4 +8,4 @@ Name = "Developed"
 BuildLabel = "[D]eveloped"
 Key = 68
 Color = Color( 0.372549, 0.372549, 0.372549, 1 )
-HeatGeneration = 0.5
+HeatGeneration = 0.1
diff --git a/half-earth/scripts/WorldGrid.cs b/half-earth/scripts/WorldGrid.cs
index 6083638..6a7b01a 100644
--- a/half-earth/scripts/WorldGrid.cs
+++ b/half-earth/scripts/WorldGrid.cs
@@ -18,7 +18,7 @@ public class WorldGrid : Node2D
 
     private float[] _nextValues = null;
 
-    private struct TileView
+    public struct TileView
     {
         public ShaderMaterial material;
     }
@@ -32,21 +32,6 @@ public class WorldGrid : Node2D
         GenerateCanvasItems(_tileGrid);
     }
 
-    public override void _Process(float delta)
-    {
-        base._Process(delta);
-
-        // TODO: this is a rendering concern that should be performed
-        // by the heat overlay
-        for (int i = 0; i < _tileGrid.Count; i++)
-        {
-            var tile = _tileGrid[i];
-            var view = _tileViews[i];
-
-            view.material.SetShaderParam("t", tile.temperature);
-        }
-    }
-
     #region Positioning
     public void GetGridPos(Vector2 position, out int x, out int y)
     {
@@ -116,13 +101,15 @@ public class WorldGrid : Node2D
         }
     }
 
+    public ShaderMaterial GetTileMaterial(int idx) => _tileViews[idx].material;
+
+    #region Simulation
     public void _on_Clock_OnTick(int ticks)
     {
         GenerateHeat();
         Diffuse();
     }
 
-    #region Simulation
     private float TransferHeat(float t0, float alpha, float nx, float ny, float px, float py)
     {
         float d2tdx2 = nx - 2 * t0 + px;
diff --git a/half-earth/scripts/overlays/HeatOverlay.cs b/half-earth/scripts/overlays/HeatOverlay.cs
new file mode 100644
index 0000000..42d1ca4
--- /dev/null
+++ b/half-earth/scripts/overlays/HeatOverlay.cs
@@ -0,0 +1,22 @@
+using Godot;
+using System;
+
+public class HeatOverlay : Overlay
+{
+    [Export]
+    private Color _coldColor;
+
+    [Export]
+    private Color _hotColor;
+
+    public override void Initialise(Tile tile, ShaderMaterial material)
+    {
+        material.SetShaderParam("lowColor", _coldColor);
+        material.SetShaderParam("highColor", _hotColor);
+    }
+
+    public override void Process(Tile tile, ShaderMaterial material)
+    {
+        material.SetShaderParam("t", tile.temperature);
+    }
+}
diff --git a/half-earth/scripts/overlays/NoOverlay.cs b/half-earth/scripts/overlays/NoOverlay.cs
new file mode 100644
index 0000000..f92d2de
--- /dev/null
+++ b/half-earth/scripts/overlays/NoOverlay.cs
@@ -0,0 +1,17 @@
+using Godot;
+
+public class NoOverlay : Overlay
+{
+    // in this view we want to draw tiles with their normal colour
+
+    public override void Initialise(Tile tile, ShaderMaterial material)
+    {
+        var type = tile.type;
+        material.SetShaderParam("lowColor", type.Color);
+        material.SetShaderParam("t", 0);
+    }
+
+    public override void Process(Tile tile, ShaderMaterial material)
+    {
+    }
+}
\ No newline at end of file
diff --git a/half-earth/scripts/overlays/Overlay.cs b/half-earth/scripts/overlays/Overlay.cs
new file mode 100644
index 0000000..d395c09
--- /dev/null
+++ b/half-earth/scripts/overlays/Overlay.cs
@@ -0,0 +1,7 @@
+using Godot;
+
+public abstract class Overlay : Resource
+{
+    public abstract void Initialise(Tile tile, ShaderMaterial material);
+    public abstract void Process(Tile tile, ShaderMaterial material);
+}
\ No newline at end of file
diff --git a/half-earth/scripts/overlays/Overlays.cs b/half-earth/scripts/overlays/Overlays.cs
new file mode 100644
index 0000000..85c844d
--- /dev/null
+++ b/half-earth/scripts/overlays/Overlays.cs
@@ -0,0 +1,76 @@
+using Godot;
+using System;
+
+public class Overlays : Node
+{
+    [Export]
+    private KeyList _cycleOverlayKey;
+
+    [Export]
+    private Resource[] _overlayResources;
+    private Overlay[] _overlays;
+    private int _overlayIdx = 0;
+    private Overlay Current => _overlays[_overlayIdx];
+
+    [Export]
+    private Resource _tileGridResource;
+    private TileGrid _tileGrid;
+
+    [Export]
+    private NodePath _worldGridPath;
+    private WorldGrid _worldGrid;
+
+    public override void _Ready()
+    {
+        _tileGrid = (TileGrid)_tileGridResource;
+        _overlays = new Overlay[_overlayResources.Length];
+        for (int i = 0; i < _overlayResources.Length; i++)
+        {
+            _overlays[i] = (Overlay)_overlayResources[i];
+        }
+        _worldGrid = GetNode<WorldGrid>(_worldGridPath);
+    }
+
+    public override void _Input(InputEvent @event)
+    {
+        base._Input(@event);
+
+        if (!(@event is InputEventKey keyEvent))
+            return;
+
+        if (!(keyEvent.Pressed))
+            return;
+
+        if ((KeyList)keyEvent.Scancode != _cycleOverlayKey)
+            return;
+
+        CycleOverlay();
+    }
+
+    public override void _Process(float delta)
+    {
+        base._Process(delta);
+
+        for (int i = 0; i < _tileGrid.Count; i++)
+        {
+            var tile = _tileGrid[i];
+            var material = _worldGrid.GetTileMaterial(i);
+
+            Current.Process(tile, material);
+        }
+    }
+
+    private void CycleOverlay()
+    {
+        _overlayIdx++;
+        _overlayIdx %= _overlays.Length;
+
+        for (int i = 0; i < _tileGrid.Count; i++)
+        {
+            var tile = _tileGrid[i];
+            var material = _worldGrid.GetTileMaterial(i);
+
+            Current.Initialise(tile, material);
+        }
+    }
+}