This paper presents perspectives and controversies surrounding the use of milestones to assess competency in outcomes-based medical education. Global perspectives (Canada, Europe, and the United States) and developments supporting their rationales are discussed. In Canada, there is a significant movement away from conceptualizing competency based on time, and a move toward demonstration of specific competencies. The success of this movement may require complex (rather than reductionist) milestones that reflect students' progression through complexity and context and a method to narrate their journey. European countries (United Kingdom, France, and Germany) have stressed the complexity associated with time and milestones for medical students to truly achieve competence. To meet the changing demands of medicine, they view time as actually providing students with knowledge and exposure to achieve various milestones. In the United States, milestones are based on sampling throughout professional development to initiate lifelong learning. However, the use of milestones may not imply overall competence (reductionism). Milestones must be developed alongside outcomes-based curriculum with use of faculty and competency committees. The perspectives outlined in this paper underscore emerging challenges for implementing outcomes-based medical education and call for new conceptualizations of competence.