diff --git a/compiler/compiler.go b/compiler/compiler.go
index 9c5b5ac8..1216d6cf 100644
--- a/compiler/compiler.go
+++ b/compiler/compiler.go
@@ -1654,6 +1654,34 @@ func (c *Compiler) parseInstr(frame *Frame, instr ssa.Instruction) error {
 
 func (c *Compiler) parseBuiltin(frame *Frame, args []ssa.Value, callName string) (llvm.Value, error) {
 	switch callName {
+	case "append":
+		src, err := c.parseExpr(frame, args[0])
+		if err != nil {
+			return llvm.Value{}, err
+		}
+		elems, err := c.parseExpr(frame, args[1])
+		if err != nil {
+			return llvm.Value{}, err
+		}
+		srcBuf := c.builder.CreateExtractValue(src, 0, "append.srcBuf")
+		srcPtr := c.builder.CreateBitCast(srcBuf, c.i8ptrType, "append.srcPtr")
+		srcLen := c.builder.CreateExtractValue(src, 1, "append.srcLen")
+		srcCap := c.builder.CreateExtractValue(src, 2, "append.srcCap")
+		elemsBuf := c.builder.CreateExtractValue(elems, 0, "append.elemsBuf")
+		elemsPtr := c.builder.CreateBitCast(elemsBuf, c.i8ptrType, "append.srcPtr")
+		elemsLen := c.builder.CreateExtractValue(elems, 1, "append.elemsLen")
+		elemType := srcBuf.Type().ElementType()
+		elemSize := llvm.ConstInt(c.uintptrType, c.targetData.TypeAllocSize(elemType), false)
+		result := c.createRuntimeCall("sliceAppend", []llvm.Value{srcPtr, elemsPtr, srcLen, srcCap, elemsLen, elemSize}, "append.new")
+		newPtr := c.builder.CreateExtractValue(result, 0, "append.newPtr")
+		newBuf := c.builder.CreateBitCast(newPtr, srcBuf.Type(), "append.newBuf")
+		newLen := c.builder.CreateExtractValue(result, 1, "append.newLen")
+		newCap := c.builder.CreateExtractValue(result, 2, "append.newCap")
+		newSlice := llvm.Undef(src.Type())
+		newSlice = c.builder.CreateInsertValue(newSlice, newBuf, 0, "")
+		newSlice = c.builder.CreateInsertValue(newSlice, newLen, 1, "")
+		newSlice = c.builder.CreateInsertValue(newSlice, newCap, 2, "")
+		return newSlice, nil
 	case "cap":
 		value, err := c.parseExpr(frame, args[0])
 		if err != nil {
diff --git a/src/runtime/runtime.go b/src/runtime/runtime.go
index 65e89325..95176460 100644
--- a/src/runtime/runtime.go
+++ b/src/runtime/runtime.go
@@ -83,17 +83,6 @@ func memequal(x, y unsafe.Pointer, n uintptr) bool {
 	return true
 }
 
-// Builtin copy(dst, src) function: copy bytes from dst to src.
-func sliceCopy(dst, src unsafe.Pointer, dstLen, srcLen lenType, elemSize uintptr) lenType {
-	// n = min(srcLen, dstLen)
-	n := srcLen
-	if n > dstLen {
-		n = dstLen
-	}
-	memmove(dst, src, uintptr(n)*elemSize)
-	return n
-}
-
 //go:linkname sleep time.Sleep
 func sleep(d int64) {
 	sleepTicks(timeUnit(d / tickMicros))
diff --git a/src/runtime/slice.go b/src/runtime/slice.go
new file mode 100644
index 00000000..def8df37
--- /dev/null
+++ b/src/runtime/slice.go
@@ -0,0 +1,53 @@
+package runtime
+
+// This file implements compiler builtins for slices: append() and copy().
+
+import (
+	"unsafe"
+)
+
+// Builtin append(src, elements...) function: append elements to src and return
+// the modified (possibly expanded) slice.
+func sliceAppend(srcBuf, elemsBuf unsafe.Pointer, srcLen, srcCap, elemsLen lenType, elemSize uintptr) (unsafe.Pointer, lenType, lenType) {
+	if elemsLen == 0 {
+		// Nothing to append, return the input slice.
+		return srcBuf, srcLen, srcCap
+	}
+
+	if srcLen+elemsLen > srcCap {
+		// Slice does not fit, allocate a new buffer that's large enough.
+		srcCap = srcCap * 2
+		if srcCap == 0 { // e.g. zero slice
+			srcCap = 1
+		}
+		for srcLen+elemsLen > srcCap {
+			// This algorithm may be made more memory-efficient: don't multiply
+			// by two but by 1.5 or something. As far as I can see, that's
+			// allowed by the Go language specification (but may be observed by
+			// programs).
+			srcCap *= 2
+		}
+		buf := alloc(uintptr(srcCap) * elemSize)
+
+		// Copy the old slice to the new slice.
+		if srcLen != 0 {
+			memmove(buf, srcBuf, uintptr(srcLen)*elemSize)
+		}
+		srcBuf = buf
+	}
+
+	// The slice fits (after possibly allocating a new one), append it in-place.
+	memmove(unsafe.Pointer(uintptr(srcBuf)+uintptr(srcLen)*elemSize), elemsBuf, uintptr(elemsLen)*elemSize)
+	return srcBuf, srcLen + elemsLen, srcCap
+}
+
+// Builtin copy(dst, src) function: copy bytes from dst to src.
+func sliceCopy(dst, src unsafe.Pointer, dstLen, srcLen lenType, elemSize uintptr) lenType {
+	// n = min(srcLen, dstLen)
+	n := srcLen
+	if n > dstLen {
+		n = dstLen
+	}
+	memmove(dst, src, uintptr(n)*elemSize)
+	return n
+}
diff --git a/testdata/slice.go b/testdata/slice.go
index 215c213e..c5e4a330 100644
--- a/testdata/slice.go
+++ b/testdata/slice.go
@@ -8,8 +8,32 @@ func main() {
 	printslice("bar", bar)
 	printslice("foo[1:2]", foo[1:2])
 	println("sum foo:", sum(foo))
+
+	// copy
 	println("copy foo -> bar:", copy(bar, foo))
 	printslice("bar", bar)
+
+	// append
+	var grow []int
+	printslice("grow", grow)
+	grow = append(grow, 42)
+	printslice("grow", grow)
+	grow = append(grow, -1, -2)
+	printslice("grow", grow)
+	grow = append(grow, foo...)
+	printslice("grow", grow)
+	grow = append(grow)
+	printslice("grow", grow)
+	grow = append(grow, grow...)
+	printslice("grow", grow)
+
+	// append string to []bytes
+	bytes := append([]byte{1, 2, 3}, "foo"...)
+	print("bytes: len=", len(bytes), " cap=", cap(bytes), " data:")
+	for _, n := range bytes {
+		print(" ", n)
+	}
+	println()
 }
 
 func printslice(name string, s []int) {
diff --git a/testdata/slice.txt b/testdata/slice.txt
index d598d432..81973655 100644
--- a/testdata/slice.txt
+++ b/testdata/slice.txt
@@ -4,3 +4,10 @@ foo[1:2]: len=1 cap=3 data: 2
 sum foo: 12
 copy foo -> bar: 3
 bar: len=3 cap=5 data: 1 2 4
+grow: len=0 cap=0 data:
+grow: len=1 cap=1 data: 42
+grow: len=3 cap=4 data: 42 -1 -2
+grow: len=7 cap=8 data: 42 -1 -2 1 2 4 5
+grow: len=7 cap=8 data: 42 -1 -2 1 2 4 5
+grow: len=14 cap=16 data: 42 -1 -2 1 2 4 5 42 -1 -2 1 2 4 5
+bytes: len=6 cap=6 data: 1 2 3 102 111 111