diff --git a/python/mozbuild/mozbuild/base.py b/python/mozbuild/mozbuild/base.py
index 16dbb2e4709a..3d5a24fa849c 100644
--- a/python/mozbuild/mozbuild/base.py
+++ b/python/mozbuild/mozbuild/base.py
@@ -5,6 +5,7 @@
 from __future__ import print_function, unicode_literals
 
 import logging
+import multiprocessing
 import os
 import subprocess
 import sys
@@ -159,8 +160,8 @@ class MozbuildObject(ProcessExecutionMixin):
         if filename:
             args.extend(['-f', filename])
 
-        #if allow_parallel:
-        #    args.append('-j%d' % self.settings.build.threads)
+        if allow_parallel:
+            args.append('-j%d' % multiprocessing.cpu_count())
 
         if ignore_errors:
             args.append('-k')
diff --git a/python/mozbuild/mozbuild/mach_commands.py b/python/mozbuild/mozbuild/mach_commands.py
index 69df519be550..70db2257280d 100644
--- a/python/mozbuild/mozbuild/mach_commands.py
+++ b/python/mozbuild/mozbuild/mach_commands.py
@@ -17,12 +17,20 @@ from mach.decorators import (
 from mozbuild.base import MachCommandBase
 
 
+BUILD_WHAT_HELP = '''
+What to build. Can be a top-level make target or a relative directory. If
+multiple options are provided, they will be built serially. BUILDING ONLY PARTS
+OF THE TREE CAN RESULT IN BAD TREE STATE. USE AT YOUR OWN RISK.
+'''.strip()
+
+
 @CommandProvider
 class Build(MachCommandBase):
     """Interface to build the tree."""
 
     @Command('build', help='Build the tree.')
-    def build(self):
+    @CommandArgument('what', default=None, nargs='*', help=BUILD_WHAT_HELP)
+    def build(self, what=None):
         # This code is only meant to be temporary until the more robust tree
         # building code in bug 780329 lands.
         from mozbuild.compilation.warnings import WarningsCollector
@@ -52,18 +60,83 @@ class Build(MachCommandBase):
 
             self.log(logging.INFO, 'build_output', {'line': line}, '{line}')
 
-        status = self._run_make(srcdir=True, filename='client.mk',
-            line_handler=on_line, log=False, print_directory=False,
-            ensure_exit_code=False)
+        def resolve_target_to_make(target):
+            if os.path.isabs(target):
+                print('Absolute paths for make targets are not allowed.')
+                return (None, None)
 
-        self.log(logging.WARNING, 'warning_summary',
-            {'count': len(warnings_collector.database)},
-            '{count} compiler warnings present.')
+            target = target.replace(os.sep, '/')
+
+            abs_target = os.path.join(self.topobjdir, target)
+
+            # For directories, run |make -C dir|. If the directory does not
+            # contain a Makefile, check parents until we find one. At worst,
+            # this will terminate at the root.
+            if os.path.isdir(abs_target):
+                current = abs_target
+
+                while True:
+                    make_path = os.path.join(current, 'Makefile')
+                    if os.path.exists(make_path):
+                        return (current[len(self.topobjdir) + 1:], None)
+
+                    current = os.path.dirname(current)
+
+            # If it's not in a directory, this is probably a top-level make
+            # target. Treat it as such.
+            if '/' not in target:
+                return (None, target)
+
+            # We have a relative path within the tree. We look for a Makefile
+            # as far into the path as possible. Then, we compute the make
+            # target as relative to that directory.
+            reldir = os.path.dirname(target)
+            target = os.path.basename(target)
+
+            while True:
+                make_path = os.path.join(self.topobjdir, reldir, 'Makefile')
+
+                if os.path.exists(make_path):
+                    return (reldir, target)
+
+                target = os.path.join(os.path.basename(reldir), target)
+                reldir = os.path.dirname(reldir)
+
+        # End of resolve_target_to_make.
+
+        if what:
+            top_make = os.path.join(self.topobjdir, 'Makefile')
+            if not os.path.exists(top_make):
+                print('Your tree has not been configured yet. Please run '
+                    '|mach build| with no arguments.')
+                return 1
+
+            for target in what:
+                make_dir, make_target = resolve_target_to_make(target)
+
+                if make_dir is None and make_target is None:
+                    return 1
+
+                status = self._run_make(directory=make_dir, target=make_target,
+                    line_handler=on_line, log=False, print_directory=False,
+                    ensure_exit_code=False)
+
+                if status != 0:
+                    break
+        else:
+            status = self._run_make(srcdir=True, filename='client.mk',
+                line_handler=on_line, log=False, print_directory=False,
+                allow_parallel=False, ensure_exit_code=False)
+
+            self.log(logging.WARNING, 'warning_summary',
+                {'count': len(warnings_collector.database)},
+                '{count} compiler warnings present.')
 
         warnings_database.prune()
-
         warnings_database.save_to_file(warnings_path)
 
+        print('Finished building. Built files are in %s' % self.topobjdir)
+
         return status
 
     @Command('clobber', help='Clobber the tree (delete the object directory).')