A key debate in studies of unionization concerns the influence of structural characteristics of labor markets on labor organizing. Studies of national survey data reveal strong relationships between unionization and demographic, industrial, and occupational structures. By contrast, comparativists analyzing highly aggregated union density data emphasize the institutional determinants of labor organization. In this article I synthesize structural and institutional explanations of unionization in a multilevel analysis that combines social survey data and comparative institutional information from 16 countries of the Organisation for Economic Cooperation and Development. This analysis indicates that highly centralized collective bargaining and union-managed unemployment insurance schemes are associated with high and distinctively solidaristic patterns of unionization. More generally, the results suggests that institutions that displace market allocation with political control assist working-class organization in trade unions.