When I open my project in Android Studio, I always get this.
Back when Android Stu
The android studio support two "build mode". one is "legacy" and one is "gradle based". If you are using an very early version of android studio, probably you are still using the legacy build mode. You need to re-import your project using "gradle mode" then this warning will be gone.
To do so, select "file/import project", select your project root directory, and in the next dialog choose "Import project from external model" and select gradle. Then, you may choose "gradle wrapper" or use your own gradle distribution (1.8 for now[12/2013]).