diff --git a/debian/changelog b/debian/changelog
index 94e6a6f..08ab4c4 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -1,3 +1,9 @@
+ubuntu-dev-tools (0.128) UNRELEASED; urgency=low
+
+  * ubuntutools.builder: Detect missing builder and fail early.
+
+ -- Stefano Rivera <stefanor@debian.org>  Mon, 25 Jul 2011 19:36:41 +0200
+
 ubuntu-dev-tools (0.127) unstable; urgency=low
 
   * Move debian-distro-info, distro-info, and ubuntu-distro-info from
diff --git a/ubuntutools/builder.py b/ubuntutools/builder.py
index ef00e22..6a8a765 100644
--- a/ubuntutools/builder.py
+++ b/ubuntutools/builder.py
@@ -41,6 +41,12 @@ class Builder(object):
                          (os.path.basename(dsc_file), self.name))
         return returncode
 
+    def exists_in_path(self):
+        for path in os.environ.get('PATH', os.defpath).split(os.pathsep):
+            if os.path.isfile(os.path.join(path, self.name)):
+                return True
+        return False
+
     def get_architecture(self):
         return self.architecture
 
@@ -55,14 +61,13 @@ class Builder(object):
 
 
 class Pbuilder(Builder):
-    def __init__(self, command="pbuilder"):
-        Builder.__init__(self, "pbuilder")
-        self._command = command
+    def __init__(self, name="pbuilder"):
+        Builder.__init__(self, name)
 
     def build(self, dsc_file, dist, result_directory):
         _build_preparation(result_directory)
         cmd = ["sudo", "-E", "ARCH=" + self.architecture, "DIST=" + dist,
-               self._command, "--build",
+               self.name, "--build",
                "--architecture", self.architecture, "--distribution", dist,
                "--buildresult", result_directory, dsc_file]
         Logger.command(cmd)
@@ -71,7 +76,7 @@ class Pbuilder(Builder):
 
     def update(self, dist):
         cmd = ["sudo", "-E", "ARCH=" + self.architecture, "DIST=" + dist,
-               self._command, "--update",
+               self.name, "--update",
                "--architecture", self.architecture, "--distribution", dist]
         Logger.command(cmd)
         returncode = subprocess.call(cmd)
@@ -79,20 +84,19 @@ class Pbuilder(Builder):
 
 
 class Pbuilderdist(Builder):
-    def __init__(self, command="pbuilder-dist"):
-        Builder.__init__(self, "pbuilder-dist")
-        self._command = command
+    def __init__(self, name="pbuilder-dist"):
+        Builder.__init__(self, name)
 
     def build(self, dsc_file, dist, result_directory):
         _build_preparation(result_directory)
-        cmd = [self._command, dist, self.architecture,
+        cmd = [self.name, dist, self.architecture,
                "build", dsc_file, "--buildresult", result_directory]
         Logger.command(cmd)
         returncode = subprocess.call(cmd)
         return self._build_failure(returncode, dsc_file)
 
     def update(self, dist):
-        cmd = [self._command, dist, self.architecture, "update"]
+        cmd = [self.name, dist, self.architecture, "update"]
         Logger.command(cmd)
         returncode = subprocess.call(cmd)
         return self._update_failure(returncode, dist)
@@ -149,16 +153,19 @@ class Sbuild(Builder):
 
 _SUPPORTED_BUILDERS = {
     "cowbuilder": lambda: Pbuilder("cowbuilder"),
-    "cowbuilder-dist": lambda: Pbuilderdist("cowbuilderdist"),
+    "cowbuilder-dist": lambda: Pbuilderdist("cowbuilder-dist"),
     "pbuilder": lambda: Pbuilder(),
     "pbuilder-dist": lambda: Pbuilderdist(),
     "sbuild": lambda: Sbuild(),
 }
 
-def get_builder(builder):
-    if builder in _SUPPORTED_BUILDERS:
-        return _SUPPORTED_BUILDERS[builder]()
+def get_builder(name):
+    if name in _SUPPORTED_BUILDERS:
+        builder = _SUPPORTED_BUILDERS[name]()
+        if builder.exists_in_path():
+            return builder
+        Logger.error("Builder doesn't appear to be installed: %s", name)
     else:
-        Logger.error("Unsupported builder specified: %s.", builder)
+        Logger.error("Unsupported builder specified: %s.", name)
         Logger.error("Supported builders: %s",
                      ", ".join(sorted(_SUPPORTED_BUILDERS.keys())))