The increasing significance of project-based forms of organizing economic activities in many industries has stimulated considerable interest in project-based firms (PBFs) as distinctive kinds of economic actors that are seen by some as heralding a new logic of organizing. In particular, their fluid, temporary nature and membership of multiple networks, alliances, and partnerships have been construed as critical to the generation of radical innovations. However, PBFs differ considerably in a number of respects, notably the singularity of their goals and outputs and the distinctiveness and stability of work roles and task organization. At least four distinct ideal types of PBFs can be distinguished in these terms that can be expected to vary in their prevalence and importance across industrial sectors and in different kinds of societies because of differences in investor and employee commitment and coordination costs. Copyright 2006, Oxford University Press.