The source code is kept under version control, since most software projects require coordination of simultaneous code changes, made by multiple developers. Version control systems (VCS) have established themselves as mandatory to reliably coordinate, track, and comprehend changes in software systems.

There are a lot of choices of version control systems, such as Git, Subversion, Mercurial or CVS. In the last years, distributed revision control systems, particularly Git, have been widely accepted as the state-of-the-art tools. They use a so-called hash tree, or Merkle tree to store and resolve individual commits, which enables efficient diffs and merges.

Distributed VCS enables developers to work with project repositories in distributed ways, without constantly requiring a network connection. Every workstation has its own repository, which includes the full history and is eventually synchronized with the central project repository.

As of writing this book, the vast majority of software projects use Git as version control system.