Hey all, I already asked this in another forum with no result yet, so sorry for cross-posting if you already read this. I recently updated to iPhone SDK 4.0 and are no longer able to build my project for the iPhone simulator. The problem is that classes of one of my own libraries can no longer be found by ld: Code: "_OBJC_CLASS_$_Book", referenced from: objc-class-ref-to-Book in Category.o (maybe you meant: ...) ld: symbol(s) not found collect2: ld returned 1 exit status I already read some other posts about similar problems, like e.g. here. "Unfortunately", my problem has nothing to do with a SDK framework, but with my own lib, so the solutions mentioned there are not applicable here. The library is included under "Link Binary With Libraries" of my target, library search paths are correct (checked this with XCode and with a plain text editor directly in my project file) and correctly added to the gcc command. I'm using iPhone Simulator 3.2 as Base SDK, but the same problem occurs with SDK 4.0. Again, this works with my device target but not with my simulator target. It worked with both, though, before I updated from iPhone SDK 3.2 to 4.0. I was also able to reproduce this error by creating a new project with only one file and the linked lib. However, the lib should be okay as it worked with simulator 3.2 before I updated XCode and my SDK. More Facts: Mac OS X 10.6.3, XCode 3.2.3 64-Bit, iPhone OS 3.2, GCC 4.2 Any help would be highly appreciated. EDIT: After reinstalling SDK 3.2, everything worked as expected again. It must be noted, that the SDKs in folder /Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs are overwritten with each installation. I then tried to reinstall SDK 4.0 again and the same error as above occurred. I didn't change anything in my project and always used 3.2 as Base SDK. The problem can therefore possibly be narrowed down to the following options: Apple changed sth. in the Simulator SDK 3.2 that causes the problem a change in XCode 3.2.3 causes the problem wrong/missing compiler flags while building my own library (those flags, however, would only cause problems with a changed SDK or the changed IDE) I also did a folder diff on /Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator3.2.sdk for both the old and the new version of this folder. It turned out that there are actually minor changes within the "same" SDK. I still think it's an IDE/compiler flag problem, though. Anybody any ideas?