diff --git a/addons/terrain-editing/terrain_editing_plugin.gd b/addons/terrain-editing/terrain_editing_plugin.gd
index 54d8cf8..979fe63 100644
--- a/addons/terrain-editing/terrain_editing_plugin.gd
+++ b/addons/terrain-editing/terrain_editing_plugin.gd
@@ -20,6 +20,7 @@ func _make_visible(visible: bool) -> void:
 		if not controls:
 			var controls_scene = load("res://terrain/editing/TerrainEditingControls.tscn")
 			controls = controls_scene.instantiate()
+			controls.EditorUndoRedo = get_undo_redo()
 		add_control_to_container(CONTAINER, controls)
 	elif controls and controls.get_parent():
 		remove_control_from_container(CONTAINER, controls)
diff --git a/terrain/Tile.cs b/terrain/Tile.cs
index 994a081..4523288 100644
--- a/terrain/Tile.cs
+++ b/terrain/Tile.cs
@@ -90,6 +90,9 @@ public struct Corners<T>(T topLeft, T topRight, T bottomRight, T bottomLeft)
 	public T BottomRight = bottomRight;
 	public T BottomLeft  = bottomLeft;
 
+	public Corners(T value)
+		: this(value, value, value, value) {  }
+
 	public T this[Corner corner] {
 		readonly get => corner switch {
 			Corner.TopLeft     => TopLeft,
diff --git a/terrain/editing/TerrainEditingControls+Editing.cs b/terrain/editing/TerrainEditingControls+Editing.cs
index 3539871..1da3c38 100644
--- a/terrain/editing/TerrainEditingControls+Editing.cs
+++ b/terrain/editing/TerrainEditingControls+Editing.cs
@@ -1,5 +1,10 @@
+using System.IO;
+
 public partial class TerrainEditingControls
 {
+	// Set by the terrain editing plugin.
+	public EditorUndoRedoManager EditorUndoRedo { get; set; }
+
 	Terrain _currentTerrain = null;
 
 	Material _editToolMaterial;
@@ -76,7 +81,6 @@ public partial class TerrainEditingControls
 
 				// TODO: Handle different tool modes, such as flatten and painting.
 				// TODO: Allow click-dragging which doesn't affect already changed tiles / corners.
-				// TODO: Support undo and redo.
 				// TODO: Make mesh generation generate vertical walls between disconnected corners.
 				// TODO: Use ArrayMesh instead of ImmediateMesh.
 
@@ -84,13 +88,16 @@ public partial class TerrainEditingControls
 				if (mouse is InputEventMouseButton { ButtonIndex: MouseButton.Left, Pressed: true }) {
 					GetViewport().SetInputAsHandled();
 
-					const float AdjustHeight = 0.5f;
-					var amount = UpDownToggle.ButtonPressed ? AdjustHeight : -AdjustHeight;
+					var cornersToChange = new HashSet<(TilePos Position, Corner Corner)>();
+
+					// Raise selected tiles themselves.
+					foreach (var pos in tiles)
+						foreach (var corner2 in Enum.GetValues<Corner>())
+							cornersToChange.Add((pos, corner2));
 
 					// If the Connected toggle button is active, move "connected" corners.
 					// Connected corners are the ones that are at the same height as ones already being moved.
 					if (ConnectedToggle.ButtonPressed) {
-						var corners = new HashSet<(TilePos Position, Corner Corner)>();
 						foreach (var pos in tiles) {
 							var tile2 = terrain.GetTile(pos);
 							foreach (var corner2 in Enum.GetValues<Corner>()) {
@@ -98,30 +105,47 @@ public partial class TerrainEditingControls
 								foreach (var (neighborPos, neighborCorner) in GetNeighbors(pos, corner2)) {
 									if (tiles.Contains(neighborPos)) continue;
 									var neighborHeight = terrain.GetTile(neighborPos).Height[neighborCorner];
-									if (neighborHeight == height) corners.Add((neighborPos, neighborCorner));
+									if (neighborHeight == height) cornersToChange.Add((neighborPos, neighborCorner));
 								}
 							}
 						}
-
-						// Raise connected corners.
-						foreach (var group in corners.GroupBy(e => e.Position, e => e.Corner)) {
-							var pos = group.Key;
-							var tile2 = terrain.GetTile(pos);
-							foreach (var corner2 in group)
-								tile2.Height[corner2] += amount;
-							terrain.SetTile(pos, tile2);
-						}
 					}
 
-					// Raise selected tiles themselves.
-					foreach (var pos in tiles) {
+					const float AdjustHeight = 0.5f;
+					var flatten = ToolMode == ToolMode.Flatten;
+					var amount = flatten ? terrain.GetTile(tile).Height[corner]
+					           : UpDownToggle.ButtonPressed ? +AdjustHeight
+					                                        : -AdjustHeight;
+
+					var tilesPrevious = new List<(TilePos, Corners<float>)>();
+					var tilesChanged  = new List<(TilePos, Corners<float>)>();
+					foreach (var group in cornersToChange.GroupBy(c => c.Position, c => c.Corner)) {
+						var pos = group.Key;
 						var tile2 = terrain.GetTile(pos);
-						tile2.Height.Adjust(amount);
-						terrain.SetTile(pos, tile2);
+						tilesPrevious.Add((pos, tile2.Height));
+
+						var newHeight = tile2.Height;
+						foreach (var corner2 in group) {
+							if (flatten) newHeight[corner2]  = amount;
+							else         newHeight[corner2] += amount;
+						}
+						tilesChanged.Add((pos, newHeight));
 					}
 
-					terrain.UpdateMeshAndShape();
-					terrain.NotifyPropertyListChanged();
+					if (EditorUndoRedo is EditorUndoRedoManager undo) {
+						var name = "Modify terrain"; // TODO: Change name depending on tool mode.
+						undo.CreateAction(name, customContext: terrain, backwardUndoOps: false);
+
+						undo.AddDoMethod(this, nameof(TerrainModifyHeight), terrain, Pack(tilesChanged));
+						undo.AddDoMethod(terrain, GodotObject.MethodName.NotifyPropertyListChanged);
+						undo.AddDoMethod(terrain, nameof(Terrain.UpdateMeshAndShape));
+
+						undo.AddUndoMethod(this, nameof(TerrainModifyHeight), terrain, Pack(tilesPrevious));
+						undo.AddUndoMethod(terrain, GodotObject.MethodName.NotifyPropertyListChanged);
+						undo.AddUndoMethod(terrain, nameof(Terrain.UpdateMeshAndShape));
+
+						undo.CommitAction(true);
+					}
 				}
 
 				UpdateEditToolMesh(terrain, tiles);
@@ -131,6 +155,43 @@ public partial class TerrainEditingControls
 		}
 	}
 
+	public void TerrainModifyHeight(Terrain terrain, byte[] data)
+	{
+		foreach (var (pos, corners) in Unpack(data)) {
+			var tile = terrain.GetTile(pos);
+			tile.Height = corners;
+			terrain.SetTile(pos, tile);
+		}
+	}
+
+	static byte[] Pack(IEnumerable<(TilePos Position, Corners<float> Corners)> data) {
+		using var stream = new MemoryStream();
+		using var writer = new BinaryWriter(stream);
+		foreach (var (pos, corners) in data) {
+			writer.Write(pos.X);
+			writer.Write(pos.Y);
+			writer.Write(corners.TopLeft);
+			writer.Write(corners.TopRight);
+			writer.Write(corners.BottomRight);
+			writer.Write(corners.BottomLeft);
+		}
+		return stream.ToArray();
+	}
+
+	static IEnumerable<(TilePos Position, Corners<float> Corners)> Unpack(byte[] data)
+	{
+		using var stream = new MemoryStream(data);
+		using var reader = new BinaryReader(stream);
+		while (stream.Position < stream.Length) {
+			var x = reader.ReadInt32();
+			var y = reader.ReadInt32();
+			var corners = new Corners<float>(
+				reader.ReadSingle(), reader.ReadSingle(),
+				reader.ReadSingle(), reader.ReadSingle());
+			yield return (new(x, y), corners);
+		}
+	}
+
 	void UpdateEditToolMesh(Terrain terrain, IEnumerable<TilePos> tiles)
 	{
 		if (terrain != _currentTerrain) ClearEditToolMesh();