This document provides an overview of open source and proprietary software for libraries, including definitions, advantages and disadvantages of each, examples of open source library systems, and resources for deciding if open source is right for a particular library. It discusses how open source software can be customized, supported by the community or companies, and has faster development, but may have less support and documentation than proprietary systems. Resources listed provide information on specific open source systems like Evergreen and Koha, the FOSS4LIB organization, and articles comparing open source and proprietary options.