Lean is an open source theorem prover and programming language being developed at Microsoft Research. Lean aims to bridge the gap between interactive and automated theorem proving, by situating automated tools and methods in a framework that supports user interaction and the construction of fully specified axiomatic proofs.

The Lean project was launched by Leonardo de Moura at Microsoft Research in 2013. It is an open source project, hosted on GitHub. Lean 3 is the latest official release. Development has shifted to Lean 4, which is not ready for general use yet. Meanwhile, members of the Lean community centered around the mathematical components library mathlib have continued maintenance and development of Lean 3 as a separate community version. Further information about mathlib and the Lean 3 community releases can be found at the Lean Community website, including an online version of Lean.