This chapter examines the role that interest groups play in political systems across time and space. Many scholars define interest groups as voluntary organizations that appeal to government but do not participate in elections. In a comparative context, however, this formal definition is problematic as the form of interest representation varies across countries. An alternative suggestion is to distinguish ‘public’ and ‘private interest groups’, but the term ‘public interest’ is problematic because of its contentious nature. The chapter begins with a review of different definitions of interest groups and the problems associated with each. It then considers the legacies of competing theoretical traditions in the field, namely republicanism, pluralism, and neocorporatism. It also discusses the role of interest associations in practice, distinguishing different types of action that are available to different groups, including direct lobbying, political exchange, contentious politics, and private interest government.