The world depends on useful data, and so does your organization. You need software to store, share, manage, and process the data--software that costs as little as possible (hopefully nothing), whose workings are open to inspection, are guaranteed not to go away, and are accompanied by communities of people who understand the tools well. You need open source software. This small set of slides introduce what free and open source software is, its importance, why it thrives in communities of practitioners, and a few principles for benefitting from it. (Two new slides were added on April 3, 2016, about assessing a project's maturity.)