This document discusses open source software and its relevance for libraries. It begins with an overview of what open source software is, noting that the source code is freely accessible and that development occurs through peer collaboration. Examples are given of common open source programs used by libraries, including operating systems like Ubuntu, email clients like Thunderbird, and web browsers like Firefox. The document argues that open source aligns well with library values of open access to information and notes that commercial software poses risks like vendor lock-in that open source avoids. Overall it promotes open source as a good fit for libraries.