This document provides an overview of open source software for libraries. It begins with definitions of key open source software concepts. It then discusses types of open source software including integrated library systems, discovery services, digital libraries, and more. Next, it covers a SWOT analysis and considerations for planning, implementing, and maintaining open source software. It provides examples of applications of open source software in public, special, and academic libraries. Finally, it lists helpful resources and references on open source software for libraries.