This free online book was made possible by the generous support of Microsoft Research, who paid for it to be written and given away. During the process of writing, they made the expertise of the Lean development team available to both answer my questions and make Lean easier to use. In particular, Leonardo de Moura initiated the project and helped me get started, Chris Lovett set up the CI and deployment automation and provided great feedback as a test reader, Gabriel Ebner provided technical reviews, Sarah Smith kept the administrative side working well, and Vanessa Rodriguez helped me diagnose a tricky interaction between the source-code highlighting library and certain versions of Safari on iOS.

Writing this book has taken up many hours outside of normal working hours. My wife Ellie Thrane Christiansen has taken on a larger than usual share of running the family, and this book could not exist if she had not done so. An extra day of work each week has not been easy for my family—thank you for your patience and support while I was writing.

The online community surrounding Lean provided enthusiastic support for the project, both technical and emotional. In particular, Sebastian Ullrich provided key help when I was learning Lean's metaprogramming system in order to write the supporting code that allowed the text of error messages to be both checked in CI and easily included in the book itself. Within hours of posting a new revision, excited readers would be finding mistakes, providing suggestions, and showering me with kindness. In particular, I'd like to thank Arien Malec, Asta Halkjær From, Bulhwi Cha, Craig Stuntz, Daniel Fabian, Evgenia Karunus, eyelash, Floris van Doorn, František Silváši, Henrik Böving, Ian Young, Jeremy Salwen, Jireh Loreaux, Kevin Buzzard, Lars Ericson, Liu Yuxi, Mac Malone, Malcolm Langfield, Mario Carneiro, Newell Jensen, Patrick Massot, Paul Chisholm, Pietro Monticone, Tomas Puverle, Yaël Dillies, Zhiyuan Bao, and Zyad Hassan for their many suggestions, both stylistic and technical.