The Accreditation Council for Graduate Medical Education (ACGME) and the American Board of Medical Specialties (ABMS) collectively constitute the foundation of professional self-regulation in the United States. In February 1999, the 2 organizations approved 6 general competencies broadly relevant for all medical practice, followed by the official launch of the Outcomes Project in 2001. It was expected that the competencies would be an antidote to overspecification of accreditation standards, and that they would empower programs to create training programs grounded in meaningful outcomes in a developmental approach. As many programs can attest, the implementation of outcomes-based (eg, competency-based) medical education has been challenging. One reason has been the difficulty in implementing the competencies in both curriculum and assessment. Program leaders lacked shared mental models within their own training programs, accompanied by a lack of shared understanding nationally within disciplines. It is important to remember that 1 of the thorny problems the milestones were intended to address was the sources of unwanted and unwarranted variability in educational and, by extension, clinical outcomes. In addition, the community cannot improve at scale what cannot be measured, and prior frames and approaches to measurement were insufficient and ineffective. A key goal for milestones thus is to help improve the state and quality of measurement through better assessment in graduate medical education to facilitate the improved outcomes everyone desires. Approximately 10 years ago, conversations began on how to more effectively and meaningfully operationalize the competencies to help improve the design of residency and fellowship programs through the use of a developmental framework. In parallel, the ACGME began to explore mechanisms to move the accreditation system to a focus on outcomes using a continuous quality improvement philosophy. Developmental milestones, using narratives to describe in more descriptive terms the professional trajectories of residents, were seen as a way to move the outcomes project forward. Starting in 2007, the disciplines of internal medicine, pediatrics, and surgery began to create developmental milestones for the 6 competencies. Surgery would subsequently delay the development of their milestones focusing first on the SCORE curriculum. The ACGME began to restructure its accreditation processes in 2009, and soon after, milestone groups were constituted for all specialties. Milestone writing groups were cosponsored by the ACGME and the ABMS member certification boards. Early groups had significant latitude in developing their subcompetencies and milestones; specialties that started the process after 2010 used a standard template. Each milestone set was subjected to review by the educational community in the specialty. BOX 1 provides an overview of the purposes of the milestones across key stakeholders, and FIGURE 1 provides an example of a key driver diagram of milestones as an educational and clinical intervention. As FIGURE 1 highlights, milestones can potentially trigger a number of drivers, or mechanisms, to help enable changes in residency and fellowship education. In 2013, the milestones were officially launched in 7 core specialties (emergency medicine, internal medicine, neurological surgery, orthopaedic surgery, pediatrics, diagnostic radiology, and urology) as a formative, continuous quality improvement component of the new accreditation system. The remaining core disciplines and the majority of subspecialties implemented the milestones starting in July 2014. We have now reached an important ‘‘milestone’’ in the implementation process, and our commentary proDOI: http://dx.doi.org/10.4300/JGME-07-03-43