The Council of the European Union plays the main role in making important decisions in the European Union. The aim of this thesis is to explain the manner and procedure of decision making in the Council of the European Union, as almost every legislative document must first be adopted by the Council of the European Union to undergo further adoption procedure. In order to understand that sequence, i.e. the connection among EU bodies when it comes to decision making, this thesis is divided in several parts. The first part briefly describes the most important EU institutions, their respective compositions and roles. As regards the decision making procedure in the Council of the European Union, the institutions which have the most important roles are the European Commission and the European Parliament, whereby the European Commission proposes legislation, the European Parliament gives its opinion, and then the European Parliament and the Council of the European Union decide on the adoption of legislation. The second part of the thesis focuses on the Council of the European Union, its historical development and structure, and gives an extensive overview of the decision making procedure and manner, which is the subject of this thesis. The end of the thesis provides a review of the entire thesis and distinguishes the Council of the European Union from the Council of Europe and the European Council, each of which have entirely different roles despite their similar names.