Current approaches examining the effect of institutions on policy processes have difficulty in explaining the results of the legislative process of codecision between the European Parliament and Council within the European Union. The formal Treaty changes that gave rise to codecision have, in turn, given rise to a plethora of informal institutions, in a process that is difficult to understand 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 may, in turn, 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 have affected subsequent Treaty negotiations. Through strategic use of the relationship between formal and informal institutions, the European Parliament has been successful in advancing its interests over time and increasing its role in the legislative process.