This chapter discusses a brief history of changes in electoral rules sets the institutional context. It describes the development of the parties and the ideological spectrum of the party system and presents the social bases of the Partido Revolucionario Institucional (PRI) and its opposition are analyzed with the use of state-level electoral data. The chapter explains to trends in participation, especially in the period since the 1977 political reforms and suggests that the single-party nature of the Mexican regime is passing. It argues that the PRI is far from dead perhaps is even far from losing its hegemony, as the elections of 1985. The 1946 electoral law contributed to the PRI's hegemony in electoral politics by stifling the opposition's legal opportunities to compete in elections. The dominant party, the PRI, has been likened to a pluralist interest aggregator within the party system and to a political machine along the lines found in cities in the United States.