Many frustrating concurrency bugs arise from unsafely sharing data. But the Pony programming language rules out data races and many of these concurrency bugs at compile time, allowing the developer to build systems without worrying about them. In this talk, I’ll explain the general conditions that make sharing data unsafe. Starting from simple concepts, I’ll then explain reference capabilities, Pony’s answer to data safety, which ensure that data is shared safely without having to give up mutability and without resorting to locks. This talk will assume no familiarity with Pony.