The transformation of existing systems is an underexposed topic in large technical systems (LTS) research. Most LTS research has focused on the emergence and stabilization of systems, ending with momentum. But how is momentum overcome, and how do transformations come about? This article presents a multilevel perspective to understand such transformations, using insights from STS and evolutionary economics. The multilevel perspective is illustrated with a longitudinal case study of the Dutch highway system (1950 to 2000). © 2007 Sage Publications.