diff --git a/Upsilon/BaseTypes/UserData/ListUserData.cs b/Upsilon/BaseTypes/UserData/ListUserData.cs
index cafcc59..c063bcf 100644
--- a/Upsilon/BaseTypes/UserData/ListUserData.cs
+++ b/Upsilon/BaseTypes/UserData/ListUserData.cs
@@ -4,6 +4,7 @@ using System.Collections.Generic;
 using Upsilon.BaseTypes.Number;
 using Upsilon.BaseTypes.ScriptTypeInterfaces;
 using Upsilon.Evaluator;
+using Upsilon.Exceptions;
 using Upsilon.Text;
 
 namespace Upsilon.BaseTypes.UserData
@@ -37,9 +38,43 @@ namespace Upsilon.BaseTypes.UserData
             return List[i].ToScriptType();
         }
 
-        public void Set(Diagnostics diagnostics, TextSpan span, ScriptType index, ScriptType value)
+        public void Set(Diagnostics diagnostics, TextSpan span, ScriptType scriptIndex, ScriptType value)
         {
-            throw new System.NotImplementedException();
+            var index = -1;
+            if (scriptIndex.Type == Type.Number || scriptIndex.Type == Type.Unknown)
+            {
+                index = Convert.ToInt32(scriptIndex.ToCSharpObject());
+            }
+            else if (scriptIndex.Type == Type.String)
+            {
+                index = int.Parse(((ScriptString) scriptIndex).Value);
+            }
+            else
+            {
+                var (i, pos) = diagnostics.ScriptString.GetLinePosition(span.Start);
+                var line = diagnostics.ScriptString.GetLine(i);
+                throw new ScriptRuntimeException(
+                    $"Tried indexing a CSharp list with a value that's not an integer. Value: {scriptIndex.ToCSharpObject()}",
+                    i, pos, line);
+            }
+
+            index--;
+            if (index > List.Count)
+            {
+                if (index == List.Count)
+                {
+                    List.Add(value.ToCSharpObject());
+                }
+                else
+                {
+                    throw new Exception(
+                        $"Tried adding an item to a list at position {index}, but list is only of length {List.Count}");
+                }
+            }
+            else
+            {
+                List[index] = value.ToCSharpObject();
+            }
         }
 
         public override Type Type => Type.UserData;