diff --git a/ui/build/build.go b/ui/build/build.go
new file mode 100644
index 0000000..506ff51
--- /dev/null
+++ b/ui/build/build.go
@@ -0,0 +1,105 @@
+// Copyright 2017 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package build
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"text/template"
+)
+
+// Ensures the out directory exists, and has the proper files to prevent kati
+// from recursing into it.
+func SetupOutDir(ctx Context, config Config) {
+	ensureEmptyFileExists(ctx, filepath.Join(config.OutDir(), "Android.mk"))
+	ensureEmptyFileExists(ctx, filepath.Join(config.OutDir(), "CleanSpec.mk"))
+	ensureEmptyFileExists(ctx, filepath.Join(config.SoongOutDir(), ".soong.in_make"))
+	// The ninja_build file is used by our buildbots to understand that the output
+	// can be parsed as ninja output.
+	ensureEmptyFileExists(ctx, filepath.Join(config.OutDir(), "ninja_build"))
+}
+
+var combinedBuildNinjaTemplate = template.Must(template.New("combined").Parse(`
+builddir = {{.OutDir}}
+include {{.KatiNinjaFile}}
+include {{.SoongNinjaFile}}
+build {{.CombinedNinjaFile}}: phony {{.SoongNinjaFile}}
+`))
+
+func createCombinedBuildNinjaFile(ctx Context, config Config) {
+	file, err := os.Create(config.CombinedNinjaFile())
+	if err != nil {
+		ctx.Fatalln("Failed to create combined ninja file:", err)
+	}
+	defer file.Close()
+
+	if err := combinedBuildNinjaTemplate.Execute(file, config); err != nil {
+		ctx.Fatalln("Failed to write combined ninja file:", err)
+	}
+}
+
+const (
+	BuildNone          = iota
+	BuildProductConfig = 1 << iota
+	BuildSoong         = 1 << iota
+	BuildKati          = 1 << iota
+	BuildNinja         = 1 << iota
+	BuildAll           = BuildProductConfig | BuildSoong | BuildKati | BuildNinja
+)
+
+// Build the tree. The 'what' argument can be used to chose which components of
+// the build to run.
+func Build(ctx Context, config Config, what int) {
+	ctx.Verboseln("Starting build with args:", config.Arguments())
+	ctx.Verboseln("Environment:", config.Environment().Environ())
+
+	if inList("help", config.Arguments()) {
+		cmd := exec.CommandContext(ctx.Context, "make", "-f", "build/core/help.mk")
+		cmd.Env = config.Environment().Environ()
+		cmd.Stdout = ctx.Stdout()
+		cmd.Stderr = ctx.Stderr()
+		if err := cmd.Run(); err != nil {
+			ctx.Fatalln("Failed to run make:", err)
+		}
+		return
+	}
+
+	SetupOutDir(ctx, config)
+
+	if what&BuildProductConfig != 0 {
+		// Run make for product config
+		runMakeProductConfig(ctx, config)
+	}
+
+	if what&BuildSoong != 0 {
+		// Run Soong
+		runSoongBootstrap(ctx, config)
+		runSoong(ctx, config)
+	}
+
+	if what&BuildKati != 0 {
+		// Run ckati
+		runKati(ctx, config)
+	}
+
+	if what&BuildNinja != 0 {
+		// Write combined ninja file
+		createCombinedBuildNinjaFile(ctx, config)
+
+		// Run ninja
+		runNinja(ctx, config)
+	}
+}