The paper examines patterns of interest group collaboration at the EU level and investigates factors with the potential to influence collaborative behaviour. Ideas from the literature on collective action, interest group coalitions, American and European interest representation and new institutionalism create a framework for analysing the range of collaborative fora at the EU level and the factors driving collaboration among groups seeking to shape EU public policies. Empirical evidence drawn from a case study of collaboration by European environmental interest groups presents a number of conclusions about the nature of and reasons for collaboration. The European environmental interest groups favour informally organised, flexible coalitions in which the members decide their level of participation. The case study identifies collective and selective incentives, issues and allies (coalition partners) as factors driving collaborative behaviour. The case study also demonstrates the importance of considering the potential influence of opposition actors and the EU institutional framework on interest group collaboration.