Experts from the Trans European Policy Studies Association (TEPSA) network and their American colleagues contributed these essays to a New transatlanitic Agenda research project for the European Commission. They evaluate the impact of current and future EU enlargement on transatlantic relations and effects of political, economic and strategic changes on relations between the US and an enlarged EU of possibly 25 or more members. The direct effect of enlargement on transatlantic relations in terms of trade, investment and business activity may not be large, but the political outcomes will be important. The EU's prolonged integrtaion process has burdened it with an accretion of legal texts and a proliferation of instruments whose combine d weight treatens its own efficient governance. It is the success of the EU's internal reform which may ultimately determine the weight, reliability and compatibility of the pwoer balance across the Atlantic. Experts from the Trans European Policy Studies Association (TEPSA) network and their American colleagues contributed these essays to a New transatlanitic Agenda research project for the European Commission. They evaluate the impact of current and future EU enlargement on transatlantic relations and effects of political, economic and strategic changes on relations between the US and an enlarged EU of possibly 25 or more members.