This chapter reviews the main theoretical perspectives which focus directly or indirectly on the role of employers and unions in welfare state development. It also examines the conditions under which collective interests become organized and mobilized, and how well worker and employer interests have been organized and integrated into the overall political economy. The differences in the degree to which welfare states share public space are addressed; that is, the influence of the social partners on policymaking and implementation in different countries. It then explores wage bargaining, labour market policy, pension policy, and health care and shows how the interests of labour and capital are differentially affected and have varying influence across advanced economies. A final comparison of the developing societies and emerging market economies indicates that in these countries, corporatist intermediation is more fragile than in advanced economies, and organized labour and capital have less influence on employment conditions and social protection.