This chapter serves as an introductory overview of Open Source Software (OSS) and the Open Source movement. It is geared primarily for technical communicators. To provide a thorough overview, this chapter defines OSS, explains how OSS works in comparison to proprietary software, looks at the history of OSS, and examines OSS licensing types, applications in business, and overall strengths and weaknesses when compared to proprietary software. Lastly, it evaluates the practical potential of OSS as well as emerging and future trends relating to it. From this general but thorough overview the intended audience of technical communicators will gain the solid understanding needed to work successfully in an academic or professional environment where OSS continues to grow in popularity, spurring more organizations to rely on it or the Open Source ideas that have inspired and continue to drive its creation and growth.