NewYork-Presbyterian Hospital is the result of the 1998 merger of two large New York City academic medical centers, the former New York and Presbyterian Hospitals, and is affiliated with two independent medical schools, the Columbia University College of Physicians and Surgeons and the Joan and Sanford J. Weill Medical College of Cornell University. At the time of the merger, the hospital faced a number of significant challenges, chief among them the clinical integration of the two medical centers. Size, separate medical schools, geography, and different histories and cultures all presented barriers to collaboration. To bring about the needed clinical alignment, the hospital turned to service lines as a way to realize the benefits of clinical integration without forcing the consolidation of departments. In this article, members of the hospital's senior management review the thinking behind the hospital's use of the service lines, their development and operation, and the significant, positive effects they have had on volume, clinical quality, clinical efficiency, best practices, and revenue management. They discuss how the service lines were used to bring together the two clinical cultures, the impact they have had on the way the hospital is operated and managed, and why service lines have worked at NewYork-Presbyterian in contrast to other hospitals that tried and abandoned them. Service lines play an increasingly central role in the hospital's clinical and business strategies, and are being extended into the NewYork-Presbyterian health care system.