Companies usually don’t share the source code for the software they develop. While this approach is justified in software that constitutes differentiating intellectual property, proprietary development can lead to redundant development and other opportunity costs. In response, companies are increasingly open sourcing some if not all of their non-differentiating software. Given the limited academic research on this emerging topic, we bridge the gap between industry and academia by taking a practice-based approach. We investigate why and how companies engage in corporate open sourcing. We take an exploratory case study approach. Our cases are four companies with multibillion-dollar revenues each: A major e-commerce company based in Germany; a leading social networking service company based in the USA; a cloud computing software company based in the USA; and a manufacturing and media software company based in the USA. We present the resulting theory in an actionable format of state-of-the-art best practice patterns.