This chapter describes grassroots activism and the FA’s peculiar organizational structure in which 7,000 grassroots activists have a significant autonomous voice in the FA decision-making bodies. Grassroots activists’ participation in the FA occurs in Base Committees (the movement) and in the different FA factions (the coalition), two different avenues of engagement with the FA. In the former, there is no direct control of factions, leaders, or party elites because Base Committees are not involved in candidate selection. The chapter provides a thick description of the party´s structure and shows the relevance of activism in the FA and its interaction with leaders and factions in party life.