Informal institutions influence all aspects of international business (IB), but they have received limited attention in the literature relative to formal institutions. This article extends prior IB work by examining the relationships between several key formal and informal institutional factors and the international strategy of MNEs. First, it examines the direct effects of formal and informal institutional effectiveness, convergence, and distance on MNE local adaptation, developing arguments and propositions for each relationship. Second, it focuses on the interactions of formal and informal institutional effectiveness and convergence to develop an eight-fold typology of market institutional configurations. This typology depicts the role that informal institutions take in a market depending on the interaction of these three institutional factors. Third, it builds on these interactions and the concept of institutional distance to develop a conceptual framework of market and MNE institutional diversity and dynamism. This framework captures the formal and informal institutional relationships within a home market, host market, parent firm, and foreign subsidiary, as well as the formal and informal institutional relationships across each of these entities over time. In so doing, this article helps advance research on informal institutions and IB, which can lead to considerable future work on the topic.