Thanks for your interest in Android Compatibility! The links below allow you to access the key documents and information.
Android 2.3
Android 2.3 is the release of the development milestone code-named Gingerbread. Android 2.3 is the current version of Android. Source code for Android 2.3 is found in the 'gingerbread' branch in the open-source tree.
Android 2.2
Android 2.2 is the release of the development milestone code-named FroYo. Source code for Android 2.2 is found in the 'froyo' branch in the open-source tree.
Android 2.1
Android 2.1 is the release of the development milestone code-named Eclair. Source code for Android 2.1 is found in the 'eclair' branch in the open-source tree. Note that for technical reasons, there is no compatibility program for Android 2.0 or 2.0.1, and new devices must use Android 2.1.
Android 1.6
Android 1.6 was the release of the development milestone code-named Donut. Android 1.6 was obsoleted by Android 2.1. Source code for Android 1.6 is found in the 'donut' branch in the open-source tree.
Compatibility Test Suite Manual
The CTS user manual is applicable to any CTS version, but CTS 2.1 R2 and beyond require additional steps to run the accessibility tests.
Older Android Versions
There is no Compatibility Program for older versions of Android, such as Android 1.5 (known in development as Cupcake). New devices intended to be Android compatible must ship with Android 1.6 or later.