This chapter first outlines the general evolution of the European Union's (EU) decision-making regime. The pattern of EU decision making has traditionally varied across policy fields to a degree that is unknown in the national constitutional law of its member states, where laws and administrative rules are usually made according to fairly standard procedures covering all policy areas. Such policy-related variation has been a constant feature throughout the history of European integration. The chapter then examines EU legislative decision making, and reviews administrative rule making by the EU institutions. The chapter then describes other relevant forms of decision making in the European Union. Finally, it shows how successive revisions of the European treaties such as Single European Act, Treaty of Maastricht, Treaty of Amsterdam, Treaty of Nice, and Treaty of Lisbon have incrementally modified the decision-making regime of the European Union.