This chapter examines the internationalization of Professional Service Firms (PSFs), outlining its drivers, varying forms, and organizational implications. It argues that conventional internationalization theory does not apply straightforwardly to PSFs. The authors identify three key sources of PSF distinctiveness-governance, clients, and knowledge-and show how these generate not only differences between PSFs and other types of organizations but also heterogeneity amongst PSFs themselves. Based on this, four different forms of PSF internationalization are identified-network, project, federal, and transnational-and the authors note that scholarly interest has mostly focused on the last two of these. The chapter highlights change towards the transnational model as an underlying theme in PSF research. It finds little convincing evidence that this model has been successfully implemented and it is argued that, in general, PSFs are better understood as federal structures controlled by a few powerful offices than as transnational enterprises.