diff --git a/src/source/initializing.md b/src/source/initializing.md
index b4208f9..439b83f 100644
--- a/src/source/initializing.md
+++ b/src/source/initializing.md
@@ -24,10 +24,22 @@
 
 For an overview of the entire code-review and code-update process, see [Life of a Patch](life-of-a-patch.html).
 
+# Choosing a Branch #
 
+Some of the requirements for your build environment are determined by which
+version of the source code you plan to compile. See
+[Build Numbers](build-numbers.html) for a full listing of branches you may
+choose from. You may also choose to download and build the latest source code
+(called "master"), in which case you will simply omit the branch specification
+when you initialize the repository.
+
+Once you have selected a branch, follow the appropriate instructions below to
+set up your build environment.
 
 # Setting up a Linux build environment #
 
+These instructions apply to all branches, including master.
+
 The Android build is routinely tested in house on recent versions of
 Ubuntu LTS (10.04), but most distributions should have the required
 build tools available. Reports of successes or failures on other
@@ -206,15 +218,20 @@
 
 # Setting up a Mac OS X build environment #
 
-To build the Android files in a Mac OS environment, you need an
-Intel/x86 machine running MacOS 10.6 (Snow Leopard).
+In a default installation, OS X runs on a case-preserving but case-insensitive
+filesystem. This type of filesystem is not supported by git and will cause some
+git commands (such as "git status") to behave abnormally. Because of this, we
+recommend that you always work with the AOSP source files on a case-sensitive
+filesystem. This can be done fairly easily using a disk image, discussed below.
 
-Android must be built on a case-sensitive file system because the sources contain files that differ only in case. We recommend that you build Android on a partition that has been formatted with the journaled file system HFS+.  HFS+ is required to successfully build Mac OS applications such as the Android Emulator for OS X.
+Once the proper filesystem is available, building the master branch in a modern
+OS X environment is very straightforward. Earlier branches, including ICS,
+require some additional tools and SDKs.
 
-## Creating a case sensitive disk image ##
+### Creating a case-sensitive disk image ###
 
-If you want to avoid partitioning/formatting your hard drive, you can use
-a case-sensitive disk image instead. To create the image, launch Disk
+You can create a case-sensitive filesystem within your existing OS X environment
+using a disk image. To create the image, launch Disk
 Utility and select "New Image".  A size of 25GB is the minimum to
 complete the build, larger numbers are more future-proof. Using sparse images
 saves space while allowing to grow later as the need arises. Be sure to select
@@ -231,9 +248,25 @@
 
 Once mounted, you'll do all your work in the "android" volume. You can eject it (unmount it) just like you would with an external drive.
 
-## Installing required packages ##
+## Master branch ##
 
-- Install XCode from [the Apple developer site](http://developer.apple.com/).
+To build the latest source in a Mac OS environment, you will need an Intel/x86
+machine running MacOS 10.6 (Snow Leopard) or MacOS 10.7 (Lion), along with Xcode
+4.2 (Apple's Developer Tools). Although Lion does not come with a JDK, it should
+install automatically when you attempt to build the source.
+
+The remaining sections for Mac OS X only apply to those who wish to build
+earlier branches.
+
+## Branch 4.0.x and all earlier branches ##
+
+To build android-4.0.x and earlier branches in a Mac OS environment, you need an
+Intel/x86 machine running MacOS 10.5 (Leopard) or MacOS 10.6 (Snow Leopard). You
+will need the MacOS 10.5 SDK.
+
+### Installing required packages ###
+
+- Install Xcode from [the Apple developer site](http://developer.apple.com/).
 We recommend version 3.1.4 or newer, i.e. gcc 4.2.
 Version 4.x could cause difficulties.
 If you are not already registered as an Apple developer, you will have to
@@ -255,7 +288,7 @@
 
         $ POSIXLY_CORRECT=1 sudo port install bison
 
-## Reverting from make 3.82 ##
+### Reverting from make 3.82 ###
 
 For versions of Android before ICS, there is a bug in gmake 3.82 that prevents android from building.  You can install version 3.81 using MacPorts by taking the following steps:
 
@@ -279,7 +312,7 @@
 
         $ sudo port install gmake @3.81
 
-## Setting a file descriptor limit ##
+### Setting a file descriptor limit ###
 
 On MacOS the default limit on the number of simultaneous file descriptors open is too low and a highly parallel build process may exceed this limit.