Participation of firms in Open Source Software (OSS) development is steadily increasing. In fact, a substantial part of OSS projects today are developed in informal collaboration between firms and a community of voluntary contributors. As more and more firms are active in OSS, acquisitions of firms active in OSS development occur increasingly often. Yet, despite the economic and practical importance of OSS, research has so far overlooked this phenomenon.  This dissertation explores this phenomenon of acquisitions of firms active in OSS development. Michael Vetter examines the role of OSS in the pre-acquisition phase and the impact of acquisitions on OSS development in the post-acquisition phase. Using qualitative and quantitative research methods, the author examines acquisition motives, acquisition likelihood, timing of acquisitions, and outcomes of acquisitions and discuss mechanisms behind variations across acquisitions. The findings from the studies contribute to research on acquisitions and OSS development while also providing insights into how to leverage OSS in target search, selection and evaluation processes as well as how to effectively manage acquisitions of firms active in OSS and the communities around them.