This paper investigates the possibility to export the free-libre open source model that has proved successful in the case of software in other sectors. We propose a general definition of open source innovation that relies on two pillars: Openness and interactivity. First, the produced knowledge must remain open, i.e. it must be made available to all without discrimination. Second, actors of the innovation process must develop ongoing interactions in order to sustain a "bazaar" mode of knowledge production (Raymond, 1999). Open source innovation is therefore very different from open innovation (Chesbrough, 2003). We also discuss the possibility to use intellectual property rights, and patents in particular, in a copyleft way in order to secure open source innovation. Then, we investigate the contexts in which open source innovation might be successful. We argue that it may be especially promising when envisaged as providing an upstream platform of open knowledge in which firms can tap in order to develop downstream applications. We conclude by presenting two examples that fit our definition of open source innovation: The case of open source biology (Hope, 2008) and the case of creative industries (Bach et al., 2010).