Mixed delivery of public services is gaining increasing attention as a way for public managers to avoid deciding between a purely public and purely private delivery system. The unusual coexistence of public and private operators in the urban bus market in Barcelona provides an interesting context in which to analyze the challenges and opportunities posed by this system. Competition for concessions among private operators and the regulation of concessionaires generate incentives to improve efficiency and quality. Furthermore, partial privatization increases the efficiency and feasibility of public operators. In fact, competitive bidding is effective in disciplining private operators and increasing the regulators' bargaining power over both public and private firms. The reform implemented in Barcelona offers an interesting insight into all metropolitan areas that are in a position to create a number of separate concessions large enough to benefit from economies of density.