One of the coolest features of Go is goroutines, and the fact that concurrency is part of the language. But concurrency normally requires synchronization, so... the sync package comes to the rescue. The sync package provides the main primitives of synchronization to use in tandem with the go concurrency, but... how does the sync package work? How are primitives like WaitGroup, Once, or Mutex built? We are going to explore that in this talk.