AbstractThis paper describes how Systems Engineering methods have been applied for the specification and interface management of the Dutch high‐speed line project (HSL‐Zuid), and the lessons which have been learned from this experience. This project is complex both in size and in the number of internal and external interfaces. The management of these interfaces is a challenge for which Systems Engineering provides a good approach, but the application is different from the applications in other industries. The focus of this paper is on the interface management of Design & Construct type contracts with high level functional specifications.