Student-centered problem-based learning (PBL) has been implemented as an alternative curriculum by several medical schools in the United States and elsewhere. Recently, attempts to integrate PBL with lecture-based curricula have created "hybrid" curricula with varying amounts of the philosophical underpinnings of student-centered PBL. Greater clinical and research demands on faculty time threaten to diminish the use of PBL in existing curricula, whereas opportunities for expansion of PBL in medical education are being created by community-based and interdisciplinary education programs.