Rules issued by the European Commission, based on powers delegated by the Council of Ministers and the European Parliament, constitute the vast majority of all EU rules. They regulate the daily operation of common policies in all areas. Because the devil is often in the details, Commission rules are tightly controlled by the member states. This traditionally takes place in the so-called comitology system, which is a system of 200–300 member state committees set up to control and approve draft Commission rules. Comitology dates back to the early 1960s, when the Common Agricultural Policy was introduced. The institutional setup of the comitology system is a four-tiered structure composed of Treaty rules, framework rules, daily legislation, and the formal and informal working practices in the individual comitology committees. The Treaty of Lisbon gave the comitology system a major overhaul and introduced new types of Commission rules, delegated acts, and implementing acts. Research on comitology has focused on the purpose and design of the system and its daily workings. Relevant research questions for future studies include the legislative choice between delegated and implementing acts, the daily workings of the comitology committees, lobbying of comitology committees by interest groups, introduction of comitology through the back door in the delegated acts system, and the relationship between comitology and the new rule-making role of European agencies.