In this article we seek to show that treaty reform is best seen as a process, and that we are witnessing a process of constitutionalization. We challenge the distinction between day-to-day politics and the high politics of treaty reform, demonstrating that high politics approaches are unable to take the significant role of non-governmental actors into consideration. While the European Commission's impact on the Maastricht Treaty was fairly limited, particularly concerning Political Union issues, we conclude that the Commission's impact on the Amstrerdam Treaty has been considerable. In fact, such an impact is not surprising given the Commission's technical expertise and its close cooperation with both the Council Secretariat and the Presidency of the Council. It is only if the impact of non-governmental actors, such as the Commission (and the Council Secretariat), is assumed to be negligible and therefore left unexamined that our findings are surprising. In this way, the article contributes to criticism of intergovernmental approaches to European integration.