Research on professional service firms (PSFs) did not come off the ground until recently. This lack of attention is surprising, given their integral role in contemporary knowledge-based economies. In this dissertation, I focus on two professional industries: law and accounting. Historically, these industries were infused with a dominant professional logic, with its corresponding Professional Partnership configuration as dominant form of enterprise organization. However, changes in economic and social trends, government policies, and client preferences have led to the spread of a commercial ethos in the professions, with the corresponding configuration of the Managed Professional Business. This shift from the professional to the commercial logic is the point of departure for this dissertation. But what are the effects of this changing logic from professionalism to commercialism on PSFs? This dissertation shows, first, that the shift from professionalism to commercialism is not complete for all PSFs, as many professionals in mid-sized firms resist this new logic. Second, although in conflict with the logic of professionalism and the corresponding organizational practices, novel-to-context practices of strategy formation and formal governance do contribute to the performance of PSFs. Yet they do so at a cost, third, as they increase misconduct of lawyers which in Professional Partnerships were effectively remedied by collegial controls. Fourth, PSFs face a choice about the corporate objective function they pursue, as my final study suggests that combining professional and commercial logics in a single configuration is seemingly impossible