This document provides an overview of how Swift and Objective-C interact when used together. It discusses how Swift code can call Objective-C code using objc_msgSend(), how method selectors are just strings, how to add the @objc attribute to expose Swift code to Objective-C, how to call private Objective-C methods from Swift, and how to use bridging headers to simplify importing Objective-C interfaces. The document also briefly mentions namespace collision protection in Swift extensions and debugging techniques like setting breakpoints on Objective-C method names in Swift code.