Foreword ======== Why3 is a platform for deductive program verification. It provides a rich language for specification and programming, called WhyML, and relies on external theorem provers, both automated and interactive, to discharge verification conditions. Why3 comes with a standard library of logical theories (integer and real arithmetic, Boolean operations, sets and maps, etc.) and basic programming data structures (arrays, queues, hash tables, etc.). A user can write WhyML programs directly and get correct-by-construction OCaml programs through an automated extraction mechanism. WhyML is also used as an intermediate language for the verification of C, Java, or Ada programs. Why3 is a complete reimplementation of the former Why platform :cite:`filliatre07cav`. Among the new features are: numerous extensions to the input language, a new architecture for calling external provers, and a well-designed API, allowing to use Why3 as a software library. An important emphasis is put on modularity and genericity, giving the end user a possibility to easily reuse Why3 formalizations or to add support for a new external prover if wanted. Availability ~~~~~~~~~~~~ Why3 project page is https://www.why3.org/. The latest release is available there, in source format, together with this documentation and several examples. Why3 is also distributed under the form of an OPAM package and a Debian package. Why3 is distributed as open source and freely available under the terms of the GNU LGPL 2.1. See the file :file:`LICENSE`. See the file :file:`INSTALL.md` for quick installation instructions, and :numref:`sec.install` of this document for more detailed instructions. Contact ~~~~~~~ There is a public mailing list for users’ discussions: https://groupes.renater.fr/sympa/info/why3-club. Report any bug to the Why3 Bug Tracking System: https://gitlab.inria.fr/why3/why3/issues. Acknowledgements ~~~~~~~~~~~~~~~~ We gratefully thank the people who contributed to Why3, directly or indirectly: Stefan Berghofer, Sylvie Boldo, Martin Clochard, Simon Cruanes, Sylvain Dailler, Xavier Denis, Clément Fumex, Léon Gondelman, David Hauzar, Daisuke Ishii, Jacques-Henri Jourdan, Johannes Kanig, Mikhail Mandrykin, David Mentré, Benjamin Monate, Kim Nguyen, Thi-Minh-Tuyen Nguyen, Mário Pereira, Raphaël Rieu-Helft, Simāo Melo de Sousa, Asma Tafat, Piotr Trojanek, Makarius Wenzel.