This article aims at clarifying the role played by licenses within the increasingly relevant Open Source Software (OSS) phenomenon. In particular, the article explores from a theoretical point of view the comparative properties of the two main categories of OSS licenses -copyleft and non-copyleft licenses- in terms of their ability to stimulate innovation and coordination of development efforts. In order to do so, the paper relies on an incomplete contracting model. The model shows that, in spite of the fact that copyleft licenses entail the enjoyment of a narrower set of rights by both licensors and licensees, they may be preferred to non-copyleft licenses when coordination of complementary (co-specifc) investments in development is important. It thus provides a non-ideologically-based explanation for the puzzling evidence showing the dominance, in terms of diffusion, of copyleft licenses.