While there has been substantial empirical work identifying factors that influence the contribution to, and use of open source software, we have as yet little theory that identifies the key constructs and relationships that would allow us to explain and predict how open source ecosystems function. What is needed is a clearly articulated and empirically validated theory of open source ecosystems. Such a theory should: • Explain why, how, and when key resources---primarily the work of developers---are attracted to or depart from a project or an ecosystem. • Explain why, how, and when projects and ecosystems move through a life cycle, from initiation, growth, maturity, and decline and death. • Explain how decisions about use are made, and how the cumulatively influence the socio-technical position of a project within an ecosystem, and the relations of ecosystems to each other. The remainder of this chapter provides a sketch of such a theory in the form of a set of propositions, which may form the foundation for future empirical work