This document provides an overview of open source software for libraries. It defines open source as software with source code that is freely available and can be modified. Open source software has advantages like lower costs, customizability, and not depending on a single vendor for support. However, it also has disadvantages like a learning curve and lack of financial incentives for development. The document discusses several examples of open source software, operating systems, and programming languages. It also outlines criteria for open source and how the open source development model works. Overall, the document argues that while open source poses some challenges, it aligns well with library values and a pragmatic approach can help libraries benefit from open source options.