Organized medicine's modern-day professionalism movement has reached the quarter-century mark. In this article, the authors travel to an earlier time to examine the concept of profession within the work of Abraham Flexner. Although Flexner used the concept sparingly, it is clear that much of his writing on reforming medical education is grounded in his views on physicians as professionals and medicine as a profession. In the first half, the authors explore Flexner's views of profession, which were (1) empirically (as opposed to philosophically) grounded, (2) case based and comparatively framed, (3) sociological in orientation, and (4) systems based, with professionalism conceptualized as dynamic, evolving, and multidimensional. In the second half, the authors build on Flexner's systems perspective to introduce a complexity science understanding of professionalism. They define professionalism as a complex system, introduce a seven-part typology of professionalism, and explore how the organization of physician work and various flash points within medicine today reveal not one but several competing forms of professionalism at work. The authors then develop a tripartite model of professionalism with analysis at the micro, meso, and macro levels. They conclude with observations on how best to frame professionalism as a force for change in 21st-century medical education. Flexner's reforms were grounded in his vision of two particular types of professional-the physician clinician and the full-time academic physician-scientist. The authors propose reform grounded in professionalism as a complex system composed of competing types.