After successfully building for a physical device, I'm unable to build for the simulator, getting Mach-O Linker errors. The Internet speaks of both clearing project DerivedData and setting target architectures, both of which fail for me. I am using Unity 4.5.5p5. Building to a clean project folder, Xcode's architecture settings for the project are: Architectures - i386 Base SDK - iPhoneSimulator (SDK not found) Supported Platforms - iphonesimulator Valid Architectures - i386 x86_64 I can change the Base SDK to iOS 8.1 which changes the Valid Architectures to arm64 arm7 arm7s but I still get the linker errors. If I change the Valid Architectures to only arm7 or arm7s it still fails. I can also change the Architectures to Standard Architectures: Architectures - Standard Architectures (armv7 arm64) Base SDK - iPhoneSimulator (SDK not found) Supported Platforms - iOS Valid Architectures - armv7 It's the same whether I have Unity's target set to iOS6 or iOS8.1. With Xcode 5 and Unity 4.5.2, I could just build for the simulator and it'd work. How do I fix this?