Existing research on developing countries emphasizes the decisive power of teacher unions in education politics. Yet that power varies, and a full understanding of the roots of union power and the sources of cross-national variation requires deeper analysis of organizational dynamics within unions. This analysis supports four arguments. First, teachers have a range of advantages in overcoming obstacles to collective action. Second, unions are not all alike; they vary widely, from interest groups (in Chile, Brazil, and Peru) to powerful political machines (in Mexico and Ecuador). Third, the source of this variation lies in factors (e.g., influence over teacher hiring) that shift power within unions from members to leaders in political-machine unions. Fourth, analyzing the dimensions of variation helps explain the different outcomes of recent reforms to teacher careers in Latin America, especially in highlighting the staunch opposition from political-machine unions.