Organized interests, most importantly employer associations and trade unions, are major collective actors in advanced economies, shaping labour relations and social policy in multiple ways. While these interest groups seek to influence policy making via lobbying and often participate in their implementation in the social policy area, the primary role of the social partners entails the self-regulation of employment conditions. Employers (and their associations) and trade unions (and works councils) engage in collective bargaining, thereby co-determining wages and working conditions. In addition, they are often also involved in self-administration of social insurances, cooperate in employment agencies, and govern private occupational welfare schemes. In corporatist systems, governments engage with them in social concertation on economic and social policies. This chapter discusses the theoretical and comparative approaches studying the role of organized interests in respect to labour relations and social policy, mapping the main cross-national variations and trends over time.