This study of Uruguay's Frente Amplio explores four central questions for the analysis of the "new Latin American left." How did a leftist alternative emerge and grow inside an institutionalized party system? How do the socioeconomic and political factors that enabled the rise of the left in Uruguay differ from those observed in other Latin American cases? How did Frente Amplio adapt itself to profit from the opportunities that arose during the 1990s? What are the implications of the previous factors for governmental action by the FA? In answering these questions, this study integrates an analysis of the sociological and political-institutional opportunity structures consolidated during the 1990s with one of strategic partisan adaptation processes. This perspective is useful for explaining how, by 2004, Frente Amplio had built a dual support base from its historical constituency and a socially heterogeneous group alienated from traditional parties due to economic and political discontent.