diff --git a/.travis.yml b/.travis.yml
index 4b940333..d6ed9577 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -8,7 +8,7 @@ before_install:
   - echo "deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-7 main"            | sudo tee -a /etc/apt/sources.list
   - echo "deb http://ppa.launchpad.net/ubuntu-toolchain-r/test/ubuntu trusty main" | sudo tee -a /etc/apt/sources.list
   - sudo apt-get update -qq
-  - sudo apt-get install llvm-7-dev clang-7 gcc-arm-none-eabi qemu-system-arm --allow-unauthenticated -y
+  - sudo apt-get install llvm-7-dev clang-7 lld-7 gcc-arm-none-eabi qemu-system-arm --allow-unauthenticated -y
   - sudo ln -s /usr/bin/clang-7 /usr/local/bin/cc # work around missing -no-pie in old GCC version
 
 install:
@@ -27,3 +27,4 @@ script:
   - tinygo build -o test.nrf.elf      -target=nrf52840-mdk examples/blinky1
   - tinygo build -o blinky1.stm32.elf -target=bluepill     examples/blinky1
   - tinygo build -o blinky1.avr.o     -target=arduino      examples/blinky1 # TODO: avr-as/avr-gcc doesn't work
+  - tinygo build -o test.wasm.wasm    -target=wasm         examples/test
diff --git a/Dockerfile b/Dockerfile
index 61904a3a..09486d66 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -22,6 +22,6 @@ COPY --from=0 /go/src/github.com/aykevl/tinygo/targets /go/src/github.com/aykevl
 RUN wget -O- https://apt.llvm.org/llvm-snapshot.gpg.key| apt-key add - && \
     echo "deb http://apt.llvm.org/stretch/ llvm-toolchain-stretch-7 main" >> /etc/apt/sources.list && \
     apt-get update && \
-    apt-get install -y libllvm7
+    apt-get install -y libllvm7 lld-7
 
 ENTRYPOINT ["/go/bin/tinygo"]
diff --git a/docs/installation.rst b/docs/installation.rst
index 1cf8dc7b..7a3670ad 100644
--- a/docs/installation.rst
+++ b/docs/installation.rst
@@ -44,6 +44,13 @@ needs the following tools:
       ``avr-gcc``.
     * ``avrdude`` for flashing to an Arduino.
 
+WebAssembly
+~~~~~~~~~~~
+
+The WebAssembly backend only needs a special linker from the LLVM project:
+
+    * LLVM linker (``ld.lld-7``) for linking WebAssembly files together.
+
 
 Installation
 ------------
diff --git a/main.go b/main.go
index 4aa4731d..e53ec606 100644
--- a/main.go
+++ b/main.go
@@ -91,13 +91,15 @@ func Compile(pkgName, outpath, opt string, spec *TargetSpec, printIR, dumpSSA, d
 	}
 
 	// Generate output.
-	if strings.HasSuffix(outpath, ".o") {
+	outext := filepath.Ext(outpath)
+	switch outext {
+	case ".o":
 		return c.EmitObject(outpath)
-	} else if strings.HasSuffix(outpath, ".bc") {
+	case ".bc":
 		return c.EmitBitcode(outpath)
-	} else if strings.HasSuffix(outpath, ".ll") {
+	case ".ll":
 		return c.EmitText(outpath)
-	} else {
+	default:
 		// Act as a compiler driver.
 
 		// Create a temporary directory for intermediary files.
@@ -160,14 +162,13 @@ func Compile(pkgName, outpath, opt string, spec *TargetSpec, printIR, dumpSSA, d
 			}
 		}
 
-		ext := filepath.Ext(outpath)
-		if ext == ".hex" || ext == ".bin" {
+		if outext == ".hex" || outext == ".bin" {
 			// Get an Intel .hex file or .bin file from the .elf file.
-			tmppath = filepath.Join(dir, "main"+ext)
+			tmppath = filepath.Join(dir, "main"+outext)
 			format := map[string]string{
 				".hex": "ihex",
 				".bin": "binary",
-			}[ext]
+			}[outext]
 			cmd := exec.Command(spec.Objcopy, "-O", format, executable, tmppath)
 			cmd.Stdout = os.Stdout
 			cmd.Stderr = os.Stderr
@@ -386,7 +387,7 @@ func main() {
 	opt := flag.String("opt", "z", "optimization level: 0, 1, 2, s, z")
 	printIR := flag.Bool("printir", false, "print LLVM IR")
 	dumpSSA := flag.Bool("dumpssa", false, "dump internal Go SSA")
-	target := flag.String("target", llvm.DefaultTargetTriple(), "LLVM target")
+	target := flag.String("target", "", "LLVM target")
 	printSize := flag.String("size", "", "print sizes (none, short, full)")
 	nodebug := flag.Bool("no-debug", false, "disable DWARF debug symbol generation")
 	ocdOutput := flag.Bool("ocd-output", false, "print OCD daemon output during debug")
@@ -415,7 +416,11 @@ func main() {
 			usage()
 			os.Exit(1)
 		}
-		err := Build(flag.Arg(0), *outpath, *target, *opt, *printIR, *dumpSSA, !*nodebug, *printSize)
+		target := *target
+		if target == "" && filepath.Ext(*outpath) == ".wasm" {
+			target = "wasm"
+		}
+		err := Build(flag.Arg(0), *outpath, target, *opt, *printIR, *dumpSSA, !*nodebug, *printSize)
 		if err != nil {
 			fmt.Fprintln(os.Stderr, "error:", err)
 			os.Exit(1)
@@ -443,7 +448,7 @@ func main() {
 			os.Exit(1)
 		}
 		var err error
-		if *target == llvm.DefaultTargetTriple() {
+		if *target == "" {
 			err = Run(flag.Arg(0))
 		} else {
 			err = Emulate(flag.Arg(0), *target, *opt)
diff --git a/src/examples/wasm/wasm.go b/src/examples/wasm/wasm.go
new file mode 100644
index 00000000..846fa749
--- /dev/null
+++ b/src/examples/wasm/wasm.go
@@ -0,0 +1,9 @@
+package main
+
+func main() {
+}
+
+//go:export add
+func add(a, b int) int {
+	return a + b
+}
diff --git a/src/examples/wasm/wasm.html b/src/examples/wasm/wasm.html
new file mode 100644
index 00000000..d2e1bc15
--- /dev/null
+++ b/src/examples/wasm/wasm.html
@@ -0,0 +1,15 @@
+<!DOCTYPE html>
+
+<html>
+	<head>
+		<meta charset="utf-8"/>
+		<title>Go WebAssembly</title>
+		<meta name="viewport" content="width=device-width, initial-scale=1"/>
+		<script src="wasm.js" async></script>
+	</head>
+	<body>
+		<h1>WebAssembly</h1>
+		<p>Add two numbers, using WebAssembly:</p>
+		<input type="number" id="a" value="2"/> + <input type="number" id="b" value="2"/> = <input type="number" id="result" readonly/>
+	</body>
+</html>
diff --git a/src/examples/wasm/wasm.js b/src/examples/wasm/wasm.js
new file mode 100644
index 00000000..6b0aef7e
--- /dev/null
+++ b/src/examples/wasm/wasm.js
@@ -0,0 +1,55 @@
+'use strict';
+
+const WASM_URL = '../../../wasm.wasm';
+
+var wasm;
+var logLine = [];
+var memory8;
+
+var importObject = {
+  env: {
+    io_get_stdout: function() {
+      return 1;
+    },
+    resource_write: function(fd, ptr, len) {
+      if (fd == 1) {
+        for (let i=0; i<len; i++) {
+          let c = memory8[ptr+i];
+          if (c == 13) { // CR
+            // ignore
+          } else if (c == 10) { // LF
+            // write line
+            let line = new TextDecoder("utf-8").decode(new Uint8Array(logLine));
+            logLine = [];
+            console.log(line);
+          } else {
+            logLine.push(c);
+          }
+        }
+      } else {
+        console.error('invalid file descriptor:', fd);
+      }
+    },
+  },
+};
+
+function updateResult() {
+  let a = parseInt(document.querySelector('#a').value);
+  let b = parseInt(document.querySelector('#b').value);
+  let result = wasm.exports.add(a, b);
+  document.querySelector('#result').value = result;
+}
+
+function init() {
+  document.querySelector('#a').oninput = updateResult;
+  document.querySelector('#b').oninput = updateResult;
+
+  WebAssembly.instantiateStreaming(fetch(WASM_URL), importObject).then(function(obj) {
+    wasm = obj.instance;
+    memory8 = new Uint8Array(wasm.exports.memory.buffer);
+    wasm.exports.cwa_main();
+    updateResult();
+  })
+}
+
+init();
diff --git a/src/runtime/arch_arm.go b/src/runtime/arch_arm.go
index 1f15a5c5..46c2a09e 100644
--- a/src/runtime/arch_arm.go
+++ b/src/runtime/arch_arm.go
@@ -2,6 +2,10 @@
 
 package runtime
 
+import (
+	"unsafe"
+)
+
 const GOARCH = "arm"
 
 // The length type used inside strings and slices.
@@ -9,3 +13,6 @@ type lenType uint32
 
 // The bitness of the CPU (e.g. 8, 32, 64).
 const TargetBits = 32
+
+//go:extern _heap_start
+var heapStart unsafe.Pointer
diff --git a/src/runtime/arch_avr.go b/src/runtime/arch_avr.go
index 2c92967e..e7d2b5ff 100644
--- a/src/runtime/arch_avr.go
+++ b/src/runtime/arch_avr.go
@@ -2,6 +2,10 @@
 
 package runtime
 
+import (
+	"unsafe"
+)
+
 const GOARCH = "avr"
 
 // The length type used inside strings and slices.
@@ -9,3 +13,6 @@ type lenType uint16
 
 // The bitness of the CPU (e.g. 8, 32, 64).
 const TargetBits = 8
+
+//go:extern _heap_start
+var heapStart unsafe.Pointer
diff --git a/src/runtime/arch_wasm.go b/src/runtime/arch_wasm.go
index e7eaaf29..bc48ce0b 100644
--- a/src/runtime/arch_wasm.go
+++ b/src/runtime/arch_wasm.go
@@ -2,6 +2,10 @@
 
 package runtime
 
+import (
+	"unsafe"
+)
+
 const GOARCH = "wasm"
 
 // The length type used inside strings and slices.
@@ -9,3 +13,6 @@ type lenType uint32
 
 // The bitness of the CPU (e.g. 8, 32, 64).
 const TargetBits = 32
+
+//go:extern __heap_base
+var heapStart unsafe.Pointer
diff --git a/src/runtime/gc.go b/src/runtime/gc.go
index 8fd1f068..90df5859 100644
--- a/src/runtime/gc.go
+++ b/src/runtime/gc.go
@@ -6,9 +6,6 @@ import (
 	"unsafe"
 )
 
-//go:extern _heap_start
-var heapStart unsafe.Pointer
-
 var heapptr = uintptr(unsafe.Pointer(&heapStart))
 
 func alloc(size uintptr) unsafe.Pointer {
diff --git a/src/runtime/runtime_wasm.go b/src/runtime/runtime_wasm.go
new file mode 100644
index 00000000..cf4eca69
--- /dev/null
+++ b/src/runtime/runtime_wasm.go
@@ -0,0 +1,54 @@
+// +build wasm,!arm,!avr
+
+package runtime
+
+type timeUnit int64
+
+const tickMicros = 1
+
+var timestamp timeUnit
+
+// CommonWA: io_get_stdout
+func _Cfunc_io_get_stdout() int32
+
+// CommonWA: resource_write
+func _Cfunc_resource_write(id int32, ptr *uint8, len int32) int32
+
+var stdout int32
+
+func init() {
+	stdout = _Cfunc_io_get_stdout()
+}
+
+//go:export _start
+func _start() {
+	initAll()
+}
+
+//go:export cwa_main
+func cwa_main() {
+	initAll() // _start is not called by olin/cwa so has to be called here
+	mainWrapper()
+}
+
+func putchar(c byte) {
+	_Cfunc_resource_write(stdout, &c, 1)
+}
+
+func sleepTicks(d timeUnit) {
+	// TODO: actually sleep here for the given time.
+	timestamp += d
+}
+
+func ticks() timeUnit {
+	return timestamp
+}
+
+// Align on word boundary.
+func align(ptr uintptr) uintptr {
+	return (ptr + 3) &^ 3
+}
+
+func abort() {
+	// TODO
+}
diff --git a/target.go b/target.go
index 2916912a..96845759 100644
--- a/target.go
+++ b/target.go
@@ -7,6 +7,8 @@ import (
 	"path/filepath"
 	"runtime"
 	"strings"
+
+	"github.com/aykevl/go-llvm"
 )
 
 // Target specification for a given target. Used for bare metal targets.
@@ -30,6 +32,10 @@ type TargetSpec struct {
 
 // Load a target specification
 func LoadTarget(target string) (*TargetSpec, error) {
+	if target == "" {
+		target = llvm.DefaultTargetTriple()
+	}
+
 	spec := &TargetSpec{
 		Triple:      target,
 		BuildTags:   []string{runtime.GOOS, runtime.GOARCH},
@@ -54,7 +60,7 @@ func LoadTarget(target string) (*TargetSpec, error) {
 		// Expected a 'file not found' error, got something else.
 		return nil, err
 	} else {
-		// No target spec available. This is fine.
+		// No target spec available. Use the default one.
 	}
 
 	return spec, nil
diff --git a/targets/wasm.json b/targets/wasm.json
new file mode 100644
index 00000000..2da5204f
--- /dev/null
+++ b/targets/wasm.json
@@ -0,0 +1,10 @@
+{
+	"llvm-target":   "wasm32-unknown-unknown-wasm",
+	"build-tags":    ["js", "wasm"],
+	"linker":        "ld.lld-7",
+	"pre-link-args": [
+		"-flavor", "wasm",
+		"-allow-undefined-file", "targets/wasm.syms"
+	],
+	"emulator":      ["cwa"]
+}
diff --git a/targets/wasm.syms b/targets/wasm.syms
new file mode 100644
index 00000000..e48be7e7
--- /dev/null
+++ b/targets/wasm.syms
@@ -0,0 +1,2 @@
+io_get_stdout
+resource_write