React Fiber was a full re-write of React that will enable new and exciting patterns around control flow, which we've seen previewed with React Suspense. But what is a fiber? How does it relate to a coroutine? What are algebraic effects, and why do I keep hearing about them? This talk will go over these computer science topics in the context of React Fiber, to help shed some light on how React Fiber is implemented and the control flow concepts behind the new APIs.