Medical educators strive to promote the development of a sound professional identity in learners, yet it is challenging to design, implement, and sustain fair and meaningful assessments of professionalism to accomplish this goal. The authors developed and implemented a program built around a Web-based Professional Development Portfolio (PDP) to assess and document professional development in medical students at New York University School of Medicine. This program requires students to regularly document their professional development through written reflections on curricular activities spanning preclinical and clinical years. Students post reflections, along with other documents that chronicle their professional growth, to their online PDP. Students meet annually with a faculty mentor to review their portfolios, assess their professional development based on predetermined criteria, and establish goals for the coming year. In this article, the authors describe the development of the PDP and share four years of experience with its implementation. We describe the experiences and attitudes of the first students to participate in this program as reported in an annual student survey. Students' experiences of and satisfaction with the PDP was varied. The PDP has been a catalyst for honest and lively debate concerning the meaning and behavioral manifestations of professionalism. A Web-based PDP promoted self-regulation on an individual level because it facilitated narrative reflection, self-assessment, and goal setting, and it structured mentorship. Therefore, the PDP may prepare students for the self-regulation of the medical profession--a privilege and obligation under the physician's social contract with society.