Professional development (PD) is a broad, albeit vaguely defined, construct that underlies psychologists' education and training and is intrinsic to professional functioning, or professionalism, throughout psychologists' careers. This article resulted from the deliberations of a working group at the November 2002 Competencies Conference: Future Directions in Education and Credentialing in Professional Psychology. The authors propose a definition of PD and consider professionalism to be its outcome. They then focus on 2 elements of professionalism - interpersonal functioning and thinking like a psychologist-and address related development and assessment implications for training and practice. Recommendations and implications for professional psychology organizations and for training and lifelong practices of psychologists emerged for further consideration.