The Accreditation Council for Graduate Medical Education (ACGME) and the American Board of Pediatrics (ABP) have partnered to initiate the Pediatrics Milestone Project to further refine the 6 ACGME competencies and to set performance standards as part of the continued commitment to document outcomes of training and program effectiveness. Members of the Pediatrics Milestone Project Working Group searched the medical literature and beyond to create a synopsis of models and evidence for a developmental ontogeny of the elements for 52 subcompetencies. For each subcompetency, we created a series of Milestones, grounded in the literature. The milestones were vetted with the entire working group, engaging in an iterative process of revisions until reaching consensus that their narrative descriptions (1) included all critical elements, (2) were behaviorally based, (3) were properly sequenced, and (4) represented the educational continuum of training and practice. We have completed the first iteration of milestones for all subcompetencies. For each milestone, a synopsis of relevant literature provides background, references, and a conceptual framework. These milestones provide narrative descriptions of behaviors that represent the ontogeny of knowledge, skill, and attitude development across the educational continuum of training and practice. The pediatrics milestones take us a step closer to meaningful outcome assessment. Next steps include undertaking rigorous study, making appropriate modifications, and setting performance standards. Our aim is to assist program directors in making more reliable and valid judgments as to whether a resident is a "good doctor" and to provide outcome evidence regarding the program's success in developing doctors.