This chapter describes interest, pressure, or advocacy groups, which organize separately from political parties, seeking to influence public opinion and public policy. It discusses the nature of these groups and what they do, before reviewing the debate on the power of interest groups, in particular whether business has a privileged position. Studies of interest groups show the importance of these groups to the delivery of public policy, which reveals a two-way relationship between groups and the state. There is a complex pattern of governance that makes policy-making complicated and difficult terrain for governments or regional and local agencies. In today's turbulent politics, there is room for new advocacy groups to upset the equilibrium. The very unpredictability of the interest group world and the appearance of actors skilled in the use of social media may provide opportunities to influence the political agenda and to engage in more disruptive politics.