The article examines the features of the institutional and legal mechanism of the Union in the field of security and defence from its founding under the Maastricht Treaty in 1992 to the current stage under the Lisbon Treaty in 2007. The legal characteristics of the EU institutions, that deals with the defence and security of Member States is provided. In particular, the activities of the European Council, the Council of the EU, the European Parliament, as well as the EU High Representative for Foreign Affairs and Security Policy were analyzed. It is established that the institutional component of the EU's Common Security and Defence Policy is characterized by the preservation of the sovereign powers of the Member States, as the development and decision-making at the highest level takes place through intergovernmental bodies of the Union. The powers of subsidiary bodies established by decisions of the EU Council are studied. The activities of the Political and Security Committee, the EU Military Committee, the EU Military Staff, the European Defence Agency, and the Committee on Civilian Aspects of Crisis Management are analyzed. In particular, the Political and Security Committee is the central executive body of the Union and a kind of "engine" of the Common Foreign and Security Policy. Especially important is his work on crisis management and finding the most optimal solutions in this direction. The EU Military Committee is the highest military body of the European Union, which manages all military activities within the EU. The EU Staff is the Union's main military expert body, tasked with early warning, situation assessment and strategic planning in line with the European Security Strategy. The European Defence Agency is a center of cooperation in the field of defence policy between Member States, EU institutions and third countries. It is noted that the EU institutional mechanism in the field of security and defence is represented not only by purely military but also by civilian structures, among which the Committee on Civilian Aspects of Crisis Management is the key. It has been found that the above institutions have sole executive powers and are not empowered to make legally binding decisions for Member States. It is concluded that one of the trends in the development of the EU institutional mechanism in the XXI century is so-called military-civil synergy, which occurs through cooperation between military and civilian structures of the Union, whose activities are aimed at achieving systematic goals in security and defence issues.