diff --git a/OpenRa.Game/Controller.cs b/OpenRa.Game/Controller.cs
index c81820721a..a82700c6f8 100644
--- a/OpenRa.Game/Controller.cs
+++ b/OpenRa.Game/Controller.cs
@@ -93,8 +93,14 @@ namespace OpenRa.Game
 			{
 				if (!(orderGenerator is PlaceBuilding))
 				{
-					orderGenerator = new UnitOrderGenerator(
-							Game.SelectActorsInBox(Game.CellSize * dragStart, Game.CellSize * xy));
+					var newSelection = Game.SelectActorsInBox(Game.CellSize * dragStart, Game.CellSize * xy);
+					var oldSelection = (orderGenerator is UnitOrderGenerator)
+						? (orderGenerator as UnitOrderGenerator).selection : new Actor[] { }.AsEnumerable();
+
+					if (dragStart == xy)
+						orderGenerator = new UnitOrderGenerator(mi.Modifiers.HasModifier(Keys.Shift) ? oldSelection.SymmetricDifference(newSelection) : newSelection);
+					else
+						orderGenerator = new UnitOrderGenerator(mi.Modifiers.HasModifier(Keys.Shift) ? oldSelection.Union(newSelection) : newSelection);
 					
 					var voicedUnit = ((UnitOrderGenerator)orderGenerator).selection
 						.Where(a => a.traits.Contains<Unit>() 
diff --git a/OpenRa.Game/Exts.cs b/OpenRa.Game/Exts.cs
index 7ebaab3e1e..187681e1d5 100644
--- a/OpenRa.Game/Exts.cs
+++ b/OpenRa.Game/Exts.cs
@@ -1,4 +1,8 @@
 
+using System.Windows.Forms;
+using System.Collections.Generic;
+using System.Linq;
+
 namespace OpenRa.Game
 {
 	static class Exts
@@ -7,5 +11,16 @@ namespace OpenRa.Game
 		{
 			return string.Format(fmt, args);
 		}
+
+		public static bool HasModifier(this Keys k, Keys mod)
+		{
+			return (k & mod) == mod;
+		}
+
+		public static IEnumerable<T> SymmetricDifference<T>(this IEnumerable<T> xs, IEnumerable<T> ys)
+		{
+			// this is probably a shockingly-slow way to do this, but it's concise.
+			return xs.Except(ys).Concat(ys.Except(xs));
+		}
 	}
 }
diff --git a/OpenRa.Game/MainWindow.cs b/OpenRa.Game/MainWindow.cs
index 73294f573b..048393137c 100755
--- a/OpenRa.Game/MainWindow.cs
+++ b/OpenRa.Game/MainWindow.cs
@@ -88,7 +88,8 @@ namespace OpenRa.Game
 			{
 				Button = e.Button,
 				Event = MouseInputEvent.Down,
-				Location = new int2(e.Location)
+				Location = new int2(e.Location),
+				Modifiers = ModifierKeys,
 			});
 		}
 
@@ -107,7 +108,8 @@ namespace OpenRa.Game
 			{
 				Button = e.Button,
 				Event = MouseInputEvent.Move,
-				Location = new int2(e.Location)
+				Location = new int2(e.Location),
+				Modifiers = ModifierKeys,
 			});
 		}
 
@@ -119,7 +121,8 @@ namespace OpenRa.Game
 			{
 				Button = e.Button,
 				Event = MouseInputEvent.Up,
-				Location = new int2(e.Location)
+				Location = new int2(e.Location),
+				Modifiers = ModifierKeys,
 			});
 		}
 
@@ -154,6 +157,7 @@ namespace OpenRa.Game
 		public MouseInputEvent Event;
 		public int2 Location;
 		public MouseButtons Button;
+		public Keys Modifiers;
 	}
 
 	enum MouseInputEvent { Down, Move, Up };