Error when building apk - "Multiple dex files define Lcom/google/ads/Ad"

Untick the admob jar file in the "order and export" part of the "configure build path".

On another post with a problem similar to yours the only thing mentioned that you have not already considered was updating Eclipse. I don't know how or what order that you have done these "fixes" but another poster mentioned that all at once he deleted the bin directory for his project, cleaned and rebuilt the project and restarted Eclipse. After that the error disappeared.

Good Luck- Lijap

For me the error was due to 2 different versions of AdMob SDK being included accidentally. Removing one of them fixed it.

As other have noted, this problem should be due to multiple versions of Goodle's ad SDK classes being included in the app build. What I haven't seen yet is a note that the most recent Google Play Service lib includes classes that (at least) share the same names as some in the admob sdk-- for example, I came upon this error because I was trying to build with both admob sdk v.4.0.4 and the google-play-services_lib project as a referenced project; the latter contains google-play-services.jar which contains the package com.google.ads and many of the same classes as the admob sdk. Thus it appears that the google play services lib and the admob sdk cannot both be used in a given app. My solution was to remove the admob sdk entirely from the build.