diff --git a/src/EcsPipeline.cs b/src/EcsPipeline.cs
index 084665c..62bae33 100644
--- a/src/EcsPipeline.cs
+++ b/src/EcsPipeline.cs
@@ -113,7 +113,7 @@ namespace DCFApixels.DragonECS
             {
                 return (T)result;
             }
-            Throw.UndefinedException();
+            Throw.Exception("No matching runner found.");
             return default;
         }
         public bool TryGetRunner<T>(out T runner) where T : IEcsProcess
diff --git a/src/EcsRunner.cs b/src/EcsRunner.cs
index fdb6f7b..c8037c5 100644
--- a/src/EcsRunner.cs
+++ b/src/EcsRunner.cs
@@ -96,7 +96,7 @@ namespace DCFApixels.DragonECS
             {
                 if (_isInit)
                 {
-                    Throw.UndefinedException();
+                    Throw.Exception("Reinitialization.");
                 }
                 _isInit = true;
                 _source = source;
diff --git a/src/EcsWorld.cs b/src/EcsWorld.cs
index 7c176a9..7ea189a 100644
--- a/src/EcsWorld.cs
+++ b/src/EcsWorld.cs
@@ -145,7 +145,7 @@ namespace DCFApixels.DragonECS
                 if (_worlds[worldID] != null)
                 {
                     _worldIdDispenser.Release(worldID);
-                    Throw.UndefinedException();
+                    Throw.Exception("The world with the specified ID has already been created\r\n");
                 }
             }
             id = worldID;
diff --git a/src/EcsWorld.pools.cs b/src/EcsWorld.pools.cs
index dbe11b7..077be5e 100644
--- a/src/EcsWorld.pools.cs
+++ b/src/EcsWorld.pools.cs
@@ -147,7 +147,7 @@ namespace DCFApixels.DragonECS
                 .First(o => o.IsGenericType && o.GetGenericTypeDefinition() == typeof(IEcsPoolImplementation<>))
                 .GetGenericArguments()[0])
             {
-                Throw.UndefinedException();
+                Throw.Exception("A custom pool must implement the interface IEcsPoolImplementation<T> where T is the type that stores the pool.");
             }
 #pragma warning restore IL2090 // 'this' argument does not satisfy 'DynamicallyAccessedMembersAttribute' in call to target method. The generic parameter of the source method or type does not have matching annotations.
 #endif