Free energy calculations have been around for several decades, and are now a field of their own, with its massive literature and a thick jungle of algorithms and implementations. Experienced researchers have to invest a lot of time to keep up to date (not everyone does), and to newcomers, it can be very confusing. Young researchers still use introductory material from the famous books of Allen and Tildesley, and Frenkel and Smit. While very precious, those are now old (especially Allen and Tildesley) and cover many other subjects, so they give little detail about classic techniques and virtually nothing about more recent ones, such as nonequilibrium methods or Wang-Landau sampling.This book brings together a handful of recognized experts from around the globe, who try to sum up the state of the art in free energy calculations in a concise, yet in-depth manner. Newcomers to the field will appreciate the first chapter, which gives an overview and an introduction to essential concepts. The following chapters present the main families of methods in a systematic way, from transition path sampling to simplified methods designed to treat large biological systems efficiently. Depending on each author's style, the chapters have different balances of raw theory and real-world applications. Even authors writing mostly about theory try to keep an eye on practical implications. Beyond the formally exact equations, many chapters put an emphasis on finite sampling errors and methods to minimize them: in real life, that often makes the difference between free energy calculations that work and those that don't.