diff --git a/RGB.NET.Core/Groups/AbstractLedGroup.cs b/RGB.NET.Core/Groups/AbstractLedGroup.cs
index 7007152..eab6803 100644
--- a/RGB.NET.Core/Groups/AbstractLedGroup.cs
+++ b/RGB.NET.Core/Groups/AbstractLedGroup.cs
@@ -36,7 +36,7 @@ namespace RGB.NET.Core
         #region Methods
 
         /// <inheritdoc />
-        public abstract IEnumerable<Led> GetLeds();
+        public abstract IList<Led> GetLeds();
 
         /// <inheritdoc />
         public virtual void OnAttach()
diff --git a/RGB.NET.Core/Groups/ILedGroup.cs b/RGB.NET.Core/Groups/ILedGroup.cs
index 638f146..5db5e63 100644
--- a/RGB.NET.Core/Groups/ILedGroup.cs
+++ b/RGB.NET.Core/Groups/ILedGroup.cs
@@ -25,7 +25,7 @@ namespace RGB.NET.Core
         /// Gets a list containing all <see cref="Led"/> of this <see cref="ILedGroup"/>.
         /// </summary>
         /// <returns>The list containing all <see cref="Led"/> of this <see cref="ILedGroup"/>.</returns>
-        IEnumerable<Led> GetLeds();
+        IList<Led> GetLeds();
 
         /// <summary>
         /// Called when the <see cref="ILedGroup"/> is attached to the <see cref="RGBSurface"/>.
diff --git a/RGB.NET.Groups/Groups/ListLedGroup.cs b/RGB.NET.Groups/Groups/ListLedGroup.cs
index a5fed36..eb833d8 100644
--- a/RGB.NET.Groups/Groups/ListLedGroup.cs
+++ b/RGB.NET.Groups/Groups/ListLedGroup.cs
@@ -92,9 +92,10 @@ namespace RGB.NET.Groups
         {
             if (leds == null) return;
 
-            foreach (Led led in leds)
-                if ((led != null) && !ContainsLed(led))
-                    GroupLeds.Add(led);
+            lock (GroupLeds)
+                foreach (Led led in leds)
+                    if ((led != null) && !ContainsLed(led))
+                        GroupLeds.Add(led);
         }
 
         /// <summary>
@@ -111,9 +112,10 @@ namespace RGB.NET.Groups
         {
             if (leds == null) return;
 
-            foreach (Led led in leds)
-                if (led != null)
-                    GroupLeds.Remove(led);
+            lock (GroupLeds)
+                foreach (Led led in leds)
+                    if (led != null)
+                        GroupLeds.Remove(led);
         }
 
         /// <summary>
@@ -121,7 +123,11 @@ namespace RGB.NET.Groups
         /// </summary>
         /// <param name="led">The LED which should be checked.</param>
         /// <returns><c>true</c> if the LED is contained by this ledgroup; otherwise, <c>false</c>.</returns>
-        public bool ContainsLed(Led led) => (led != null) && GroupLeds.Contains(led);
+        public bool ContainsLed(Led led)
+        {
+            lock (GroupLeds)
+                return (led != null) && GroupLeds.Contains(led);
+        }
 
         /// <summary>
         /// Merges the <see cref="Led"/> from the given ledgroup in this ledgroup. 
@@ -129,9 +135,10 @@ namespace RGB.NET.Groups
         /// <param name="groupToMerge">The ledgroup to merge.</param>
         public void MergeLeds(ILedGroup groupToMerge)
         {
-            foreach (Led led in groupToMerge.GetLeds())
-                if (!GroupLeds.Contains(led))
-                    GroupLeds.Add(led);
+            lock (GroupLeds)
+                foreach (Led led in groupToMerge.GetLeds())
+                    if (!GroupLeds.Contains(led))
+                        GroupLeds.Add(led);
         }
 
         /// <inheritdoc />
@@ -139,7 +146,11 @@ namespace RGB.NET.Groups
         /// Gets a list containing the <see cref="T:RGB.NET.Core.Led" /> from this group.
         /// </summary>
         /// <returns>The list containing the <see cref="T:RGB.NET.Core.Led" />.</returns>
-        public override IEnumerable<Led> GetLeds() => GroupLeds;
+        public override IList<Led> GetLeds()
+        {
+            lock (GroupLeds)
+                return new List<Led>(GroupLeds);
+        }
 
         #endregion
     }
diff --git a/RGB.NET.Groups/Groups/RectangleLedGroup.cs b/RGB.NET.Groups/Groups/RectangleLedGroup.cs
index 1b23c26..90bb88c 100644
--- a/RGB.NET.Groups/Groups/RectangleLedGroup.cs
+++ b/RGB.NET.Groups/Groups/RectangleLedGroup.cs
@@ -105,7 +105,7 @@ namespace RGB.NET.Groups
         /// Gets a list containing all <see cref="T:RGB.NET.Core.Led" /> of this <see cref="T:RGB.NET.Groups.RectangleLedGroup" />.
         /// </summary>
         /// <returns>The list containing all <see cref="T:RGB.NET.Core.Led" /> of this <see cref="T:RGB.NET.Groups.RectangleLedGroup" />.</returns>
-        public override IEnumerable<Led> GetLeds() => _ledCache ??= RGBSurface.Instance.Leds.Where(led => led.AbsoluteLedRectangle.CalculateIntersectPercentage(Rectangle) >= MinOverlayPercentage).ToList();
+        public override IList<Led> GetLeds() => _ledCache ??= RGBSurface.Instance.Leds.Where(led => led.AbsoluteLedRectangle.CalculateIntersectPercentage(Rectangle) >= MinOverlayPercentage).ToList();
 
         private void InvalidateCache() => _ledCache = null;