Bazel is a build system open-sourced in 2015 after years of internal-only use at Google. It's main focus is efficiently building on large code bases achieving correctness by completely tracking all dependencies. BUILD files are written in a declarative style and new rules can be added via Bazel's extension language. The talk gives an overview of the basic concepts and design principles of Bazel, trying to explain why there is still room for yet another build system.