Swift relies on modules exclusively for units of code distribution and library interface. Swift’s interoperability with C, ObjectiveC, and C++ leads to a heavy use of the concept of modules in the C family of languages. Building on top of Clang & LLVM infrastructure for dependency scanning, Swift is undergoing a transition to an Explicitly-Built Modules compilation model where all dependencies are discovered upfront, pre-built, and are specified as explicit inputs to compilation. This talk will describe this approach, its benefits compared to the prior Implicit Module Loading model, lessons learned along the way, and the extensive use of Clang infrastructure to support Swift’s interoperability with modules in the C family of languages.