FS2 is a library for purely functional, effectful, and polymorphic stream processing library in the Scala programming language. Its design goals are compositionality, expressiveness, resource safety, and speed. The name is a modified acronym for Functional Streams for Scala (FSS, or FS2). FS2 is available for Scala 2.12, Scala 2.13, Scala 3, and Scala.js. FS2 is built upon two major functional libraries for Scala, Cats, and Cats-Effect. Regardless of those dependencies, FS2 core types (streams and pulls) are polymorphic in the effect type (as long as it is compatible with cats-effect typeclasses), and thus FS2 can be used with other effect libraries, such as Monix.