Commercial open source software projects are open source software projects that are owned by a single firm that derives a di- rect and significant revenue stream from the software. Commercial open source at first glance represents an economic para- dox: How can a firm earn money if it is making its product available for free as open source? This paper presents the core properties of commercial open source business models and discusses how they work. Using a commercial open source ap- proach, firms can get to market faster with a superior product at lower cost than possible for traditional competitors. The pa- per shows how these benefits accrue from an engaged and self-supporting user community. Lacking any prior comprehensive reference, this paper is based on an analysis of public statements by practitioners of commercial open source. It forges the various anecdotes into a coherent description of revenue generation strategies and relevant business functions.