How and why do institutions change? Institutions, understood as rules of behaviour constraining and facilitating social interaction, are subject to different forms and processes of change. A change may be designed intentionally on a large scale and then be followed by a period of only incremental adjustments to new conditions. But institutions may also emerge as informal rules, persist for a long time and only be formalized later. Why? The causes, processes and outcomes of institutional change raise a number of conceptual, theoretical and empirical questions. While we know a lot about the creation of institutions, relatively little research has been conducted about their transformation once they have been put into place. Attention has focused on politically salient events of change, such as the Intergovernmental Conferences of Treaty reform. In focussing on such grand events, we overlook inconspicuous changes of European institutional rules that are occurring on a daily basis. Thus, the European Parliament has gradually acquired a right of investing individual Commissioners. This has never been an issue in the negotiations of formal treaty revisions. Or, the decision-making rule(s) under which the European Parliament participates in the legislative process have drastically changed over the last decades starting from a modest consultation ending up with codecision. The book discusses various theories accounting for long-term institutional change and explores them on the basis of five important institutional rules in the European Union. It proposes typical sequences of long-term institutional change and their theorization which hold for other contexts as well, if the number of actors and their goals are clearly defined, and interaction takes place under the "shadow of the future" .