Milestones specific to orthopaedic surgical training document individual resident progress through skill development in multiple dimensions. Residents increasingly interact with and are assessed by surgeons in both academic and private practice environments. Milestones describe the skills that support competence. One of the primary goals of milestones is to provide continuous data for educational quality improvement of residency programs. They provide a dialogue between surgeons who supervise residents or fellows and the program's Clinical Competency Committee throughout a resident's education. The orthopaedic milestones were developed jointly by the Accreditation Council for Graduate Medical Education and the American Board of Orthopaedic Surgery. The working team was designed with broad representation within the specialty. The milestones were introduced to orthopaedic residencies in 2013. Orthopaedics is a 5-year training program; the first comprehensive longitudinal data set is now available for study. This summary provides historical perspective on the development of the milestones, state of current milestone implementation, attempts to establish validity, challenges with the milestones, and the development of next-generation assessment tools.