Much of the British and European literature on the role of interest groups in the policy process focuses on their participation in policy networks of various types. Possibly reflecting the original development of the policy community and policy network ‘models’ in the late 1970s, these approaches tend to emphasize stability and continuity – of both networks and policies. However, the 1980s and 1990s have witnessed much policy change and instability in most Western European states. In particular, some governments have adopted a more impositional policy style, and interest groups have learned to exploit the opportunities presented by a policy process which is increasingly characterized by multiple opportunity structures. This is especially the case following Europeanization of many policy sectors within the fifteen EU member states. The article focuses on the possible causes of policy change, including the importance of state power; changes in the behaviour of interest groups as they adjust to and exploit the opportunities presented by multi-arena policy-making; the impact of new policy fashions, reflecting knowledge and ideas which can act as a virus-like threat to existing policy communities.