The MATSim book has been officially published. It is the official reference and user documentation for MATSim. The book covers a broad range of topics; from an introduction on how to use MATSim, over a documentation of its core features, of the official extensions and of the theoretical background of MATSim, to the presentation of over 40 use cases from around the world. We would be happy to have your comments and suggestions, as this will be a living document. Kay W. Axhausen, Institut für Verkehrsplanung und Transportsysteme IVT, ETH Zürich.