In the literature on the role of agency in the policy process, relatively little attention has been devoted to how agents define policy problems. This article helps to address this gap by asking when and how policy entrepreneurs are successful in defining problems. The article rests on a framework that shows how policy entrepreneurs holding specific ideas and given a propitious socioeconomic context are able to define problems, translate those problems into new frames, and draw on those frames, while using their personal skills and political and institutional resources, to help build supportive coalitions in favor of policy change. Illustrated by a puzzling case in the field of European mobility policy, the article offers a new perspective on the role of ideas at the problem definition stage of the policy process, while providing a richer understanding of the policy entrepreneur as a driver of policy change.