The author reviews the theoretical and empirical literature to examine the traditional perception that the following trade-off exists between economic efficiency and stability in the banking system: a competitive banking system is more efficient and therefore important to growth, but market power is necessary for stability in the banking system. That this trade-off exists is not clear. Market power can have positive implications for efficiency, and the potentially negative implications of competition on stability may be manageable through prudential regulation. Neither extreme (perfect competition nor monopoly) is likely ideal. Rather, it may be optimal to facilitate an environment that promotes competitive behaviour (contestability), thereby minimizing the potential costs of market power while realizing benefits from any residual that remains. It can be very difficult to assess the contestability of a banking market. Recent work suggests that the number of banks and the degree of concentration are not, in themselves, sufficient indicators of contestability. Other factors play a strong role, including regulatory policies that promote competition, a well-developed financial system, the effects of branch networks, and the effect and uptake of technological advancements.