Reformers have promoted mixed-member electoral systems as the “best of both worlds.” In this volume, internationally recognized political scientists evaluate the ways in which the introduction of a mixed-member electoral system affects the configuration of political parties. The contributors examine several political phenomena, including cabinet post allocation, nominations, preelectoral coalitions, split-ticket voting, and the size of party systems and faction systems. Significantly, they also consider various ways in which the constitutional system—especially whether the head of government is elected directly or indirectly—can modify the incentives created by the electoral system. Part I of the book provides an in-depth comparison of Taiwan and Japan, both of which moved from single nontransferable vote systems to mixed-member majoritarian systems. These cases demonstrate that the higher the payoffs of attaining the executive office and the greater the degree of cross-district coordination required to win it, the stronger the incentives for elites to form and stay in the major parties. In such a context, a country will move rapidly toward a two-party system. In Part II, the contributors apply this theoretical logic to other countries with mixed-member systems and find that executive competition has the same effect on legislative electoral rules in countries as disparate as Thailand, the Philippines, New Zealand, Bolivia, and Russia. The findings presented here demonstrate that the success of electoral reform depends not only on the specification of new electoral rules per se but also on the political context—and especially the constitutional framework—within which such rules are embedded.