With each legislative issue, legislators have to decide whether to delegate decision-making to the executive and/or to expert bodies in order to flesh out the details of this legislation, or, alternatively, to spell out all aspects of this decision in legislation proper. The reasons why to delegate have been of prime interest to political science. The debate has concentrated on principal-agent theory to explain why politicians delegate decision-making to bureaucrats, to independent regulatory agencies, and to others actors and how to control these agents. By contrast, Changing Rules of Delegation focuses on these questions: Which actors are empowered by delegation? Are executive actors empowered over legislative actors? How do legislative actors react to theloss of power? What opportunities are there to change the institutional rules governing delegation in order to (re)gain institutional power and, with it influence over policy outcomes? The authors analyze the conditions and processes of change of the rules that delegate decision-making power to the Commission’s implementing powers under comitology. Focusing on the role of the European Parliament the authors explain why the Commission, the Council, and increasingly the Parliament, delegateddecision-making to the Commission. If they chose delegation, they still have to determine under which institutional rule comitology should operate. These rules, too, distribute power unequally among actors and therefore raise the question of how they came about in the first place and whether and how the"losers" of a rule change seek to alter the rules at a later point in time.