Is Mexico in a state of political transition from a democratic single-party system to a new system? In [Politics and Society] [in Mexico], Martin Needler maintains that the very success of the official party is leading to its erosion. He anticipates evolution either toward a two- or multiparty system that will include an internally more democratic Partido Revolucionario Institucional (PRI), or, through force, toward an undemocratic single-party system. Daniel Cosio Villegas, in his [El Sistema Politico Mexicano: Las] [Posibilidades de Cambio], argues that Mexico's most serious political problem is to contain and even reduce the excessive power concentrated in the presidency and sustained by the PRI. But, says Cosio, no single source is likely to foment democratizing reforms, and antidemocratic tendencies may become stronger. Whereas important weaknesses limit the usefulness of Needler's book for advanced analysis, the reviewer believes that Cosio's essay provides today's best discussion of Mexico's possibilities for change. 5 pp.