Abstract: Today economic activity is increasingly organized by fusing elements of hierarchies into market transactions (e.g., quasi-integration) or bringing aspects of markets within hierarchies (e.g., empowerment). These hybrid governance forms are not adequately addressed in most extant theories that envision a unidimensional continuum between markets and hierarchies, thereby ignoring true hybrids that are market-like in some ways yet hierarchy-like in others. We contribute first by creating a taxonomy of hybrid forms and second by developing a formal theory that predicts when a given hybrid form will be efficient. Our model is unique in that we consider cross-task synergies in a multi-task principal-agent model, where hybrid forms result as principals try to motivate cooperation among agents indirectly through incentives, ownership and formal authority. We conclude with a discussion of other mechanisms that might also help to understand and predict hybrid governance forms.