diff --git a/stdlib/internal/types/collections/list.codon b/stdlib/internal/types/collections/list.codon
index d44ae48c..7f7ad8a0 100644
--- a/stdlib/internal/types/collections/list.codon
+++ b/stdlib/internal/types/collections/list.codon
@@ -200,16 +200,26 @@ class List:
             i -= 1
 
     def __add__(self, other: List[T]) -> List[T]:
-        v = List[T](self.len + other.len)
-        for a in self:
-            v.append(a)
-        for a in other:
-            v.append(a)
+        n = self.len + other.len
+        v = List[T](n)
+        v.len = n
+        p = v.arr.ptr
+        str.memcpy(p.as_byte(),
+                   self.arr.ptr.as_byte(),
+                   self.len * gc.sizeof(T))
+        str.memcpy((p + self.len).as_byte(),
+                   other.arr.ptr.as_byte(),
+                   other.len * gc.sizeof(T))
         return v
 
     def __iadd__(self, other: List[T]) -> List[T]:
-        for a in other:
-            self.append(a)
+        n = self.len + other.len
+        if self.arr.len < n:
+            self._resize(n)
+        str.memcpy((self.arr.ptr + self.len).as_byte(),
+                   other.arr.ptr.as_byte(),
+                   other.len * gc.sizeof(T))
+        self.len = n
         return self
 
     def __mul__(self, n: int) -> List[T]:
diff --git a/test/core/containers.codon b/test/core/containers.codon
index ed6d58e6..db662cca 100644
--- a/test/core/containers.codon
+++ b/test/core/containers.codon
@@ -138,6 +138,24 @@ def test_list():
     assert list(test_cmp([1,2,-1], [2])) == [('EQ', False), ('NE', True), ('LT', True), ('GT', False), ('LE', True), ('GE', False)]
     assert list(test_cmp([1,2,-1], [1,2,-1,3])) == [('EQ', False), ('NE', True), ('LT', True), ('GT', False), ('LE', True), ('GE', False)]
     assert list(test_cmp([1,2,-1,3], [1,2,-1])) == [('EQ', False), ('NE', True), ('LT', False), ('GT', True), ('LE', False), ('GE', True)]
+
+    assert str([1] + [2] + [] + [3]) == '[1, 2, 3]'
+    assert ['a', 'b'] + ['x', 'y', 'z'] == ['a', 'b'] + ['x', 'y', 'z']
+    assert [(1,1), (2,2)] + [] == [(1,1), (2,2)]
+    assert [] + [(1,1), (2,2)] == [(1,1), (2,2)]
+    assert List[int]() + List[int]() == List[int]()
+    l7 = [3.14, 2.5]
+    l7 += [9.99, -1.0]
+    assert l7 == [3.14, 2.5, 9.99, -1.0]
+    l8 = []
+    l8 += [11, 22, 33]
+    assert l8 == [11, 22, 33]
+    l8 = [11, 22, 33]
+    l8 += []
+    assert l8 == [11, 22, 33]
+    l8 = List[int]()
+    l8 += List[int]()
+    assert l8 == List[int]()
 test_list()
 
 @test