I have several projects in eclipse, but some of the android projects are telling me that import android.app.Activity cannot be resolved. I looked at the Java build path and I noticed that the working projects have Android 2.1 in the libraries tab under Java build path whereas the ones that don't work have JRE System Library instead. Is there a way to use either Add JARs/Add External JARs/Add Variable/Add Library/Add Class Folder/ or Add External Class Folder to fix this? I've tried all of them and can't seem to find a way to add Android 2.1. The only other way I know to fix the errors is to delete the projects from workspace and then create new projects from an existing source.