It is common for developers to use open-source libraries and frameworks: open-source software is freely available, often robust, and flexible. Using OSS libraries makes the developer’s work easier avoiding the developers starting each time from scratch which is inefficient (long time to reach a stable software release) and lead to duplication of effort. One of the first tasks of RSE at the eScience center is to identify potential OSS libraries and use them appropriately. In Unit 1, we cover the basics of OSS libraries which hopefully will help you to make well-informed decisions about OSS libraries in your projects.