Transnational political networks play a central role in shaping the political process within the present-day European Union (EU). This process is characterised above all by a high complexity of institutional procedures and policy issues. It involves national governments and supranational institutions like the European Commission and the European Parliament (EP), but also a large number of political and societal actors from political parties to socio-economic interest groups and non-governmental organisations that have developed from the new social movements of the 1970s. As scholars of public policy first observed in relation to national political systems in the late 1980s and early 1990s, increasingly complex regulatory issues and growing domestic distributional conflicts have increased the importance of access to information, technical expertise and the ability to muster political support and create societal coalitions for policy options for influencing increasingly informal processes of coordination and decision-making. This informal coordination tends to marginalise parliaments as the traditional sites of deliberation and legislative decision-making. These conditions apply even more in the EU of twenty-seven member states, where no political actor or collective interest can easily dominate policy agendas. Indeed, national political actors and collective interests stand no realistic chance of influencing the policy-making process significantly unless they are well connected across borders in transnational political networks, from the more formalised cooperation of EU-level political parties with a general stake in EU politics to highly informal expert networks within a specific policy sector, or what Peter M. Haas first called an ‘epistemic community’.