Open source software systems have gained a lot of attention in the past few years. With the emergence of open source platforms like GitHub, developers can contribute, store, and manage their projects with ease. Large organizations like Microsoft, Google, and Facebook are open sourcing their in-house technologies in an effort to more broadly involve the community in the development of software systems. Although closed source and open source systems have been studied extensively, there has been little research on the transition from closed source to open source systems. Through this study we aim to: a) provide guidance and insights for other teams planning to open source their projects and b) to help them avoid pitfalls during the transition process. We studied six different Microsoft systems, which were recently open-sourced i.e., CoreFX, CoreCLR, Roslyn, Entity Framework, MVC, and Orleans. This paper presents the transition from the viewpoints of both Microsoft and the open source community based on interviews with eleven Microsoft developer, five Microsoft senior managers involved in the decision to open source, and eleven open-source developers. From Microsoft’s perspective we discuss the reasons for the transition, experiences of developers involved, and the transition’s outcomes and challenges. Our results show that building a vibrant community, prompt answers, developing an open source culture, security regulations and business opportunities are the factors which persuade companies to open source their products. We also discuss the transition outcomes on processes such as code reviews, version control systems, continuous integration as well as developers’ perception of these changes. From the open source community’s perspective, we illustrate the response to the open-sourcing initiative through contributions and interactions with the internal developers and provide guidelines for other projects planning to go open source.