歐洲聯盟於2004年6月18日之政府間首長會議中，就歐洲憲法條約草案達成共識，並於同年10月29日正式簽署「歐洲憲法條約」（Treaty establishing a Constitution for Europe）。回顧歐洲整合過程從功能性的政策協調，到共同政策領域的全方位擴大，以及命全決策機構形成三權分立運作，乃至於歐盟條約的修改再至歐洲憲法條約的制定等，每一步都標誌著歐盟這個政體（Polity）的轉型發展。這個已整合25個成員國，而且還會繼續擴大的政治體，不論從「主權問題」、「權力的水平與垂直分立」或「決策參與的民主程度」的視角去研析其政治型態上的意義，最終都會觸及一個核心問題：歐盟究竟是一個什麼樣的政體？本文從憲政主義出發，試圖跨越「超國家主義」和「政府間合作主義」所呈現的「體系特質」，與以「制度主義」取向的「功能型式」之間的爭論，來探究歐盟政體的性質。研究發現，歐洲憲法條約建構一政體具有「歐洲國家聯邦」（F?d?ration dEtats europ?enne）的特質。然面對法、荷兩國對歐洲憲法條約公投的挫敗，歐盟將檢討民意認同歐盟發展的傾向，這會是歐洲整合的又一次轉機。

The heads of member states of the EU signed the treaty establishing a constitution for Europe on 29 October 2004. Looking back at he history of European integration, from coordination of policies, to the common policies in all policy fields; from the institutional design of administration, legislature and judiciary in the EC and EU, to the drafting of the constitutional treaty, every step signifies the evolution of the EU’s polity. With currently 25 member states, the EU will continue to add members. No matter whether discussing the EU in terms of sovereignty, distribution of powers, or public participation and degree of democracy, we eventually have to focus on the core theme: what kind of polity is the EU? Beginning from constitutionalism, this paper tries to analyze the character of the EU, avoiding traditional views of supranationalism, intergovernmentalism from the perspective of system character, and institutionalism from the perspective of functionalism. A result of this study is summarized: the development of the EU is toward a federation of the European states (F?d?ration dEtats europ?enne). However, faced with the negative referenda of France and Netherlands, the EU will continue to be reexamined by all of the EU’s citizens. The EU is now at a turning point for European integration.