Current approaches to the understanding of institutional change in the European Union have difficulty in understanding how intergovernmental bargaining and day-to-day institutional change interact. This article develops a theoretical framework to understand this interaction, and applies it to the legislative process of codecision between European Parliament and Council. The formal Treaty changes which gave rise to codecision have in turn given rise to a plethora of informal institutions, in a process which cannot be grasped by using dominant modes of analysis. This article provides a framework for analyzing the relationship between formal and informal institutions, showing how the two may be recursively related. Formal institutional change at a particular moment in time may give rise to informal institutions, which in turn may affect the negotiation of future formal institutions. The article applies this framework to the codecision process, showing how the codecision procedure has led to the creation of informal institutions and modes of decision making, which in turn have affected subsequent Treaty negotiations. Through strategic use of the relationship between formal and informal institutions, Parliament has been successful in advancing its interests over time, and increasing its role in the legislative process.