Explore the Lean theorem prover in this comprehensive lecture by Leonardo de Moura at the Paul G. Allen School. Delve into the goals, architecture, and key features of this open-source project developed by Microsoft Research and Carnegie Mellon University. Learn how Lean bridges the gap between interactive and automated theorem proving, offering a framework for user interaction and axiomatic proof construction. Discover its applications in formalizing datatypes, algebraic structures, homotopy type theory, category theory, and analysis. Gain insights into Lean's trusted kernel based on dependent type theory, integrated development environments, and rich API. Understand the project's long-term vision and its current capabilities in the field of automated reasoning and theorem proving.