Problem-based learning (PBL) has emerged as a useful tool of epistemological reform in higher education, particularly in medical schools. Indeed, PBL has spent most of its career inducing revolutionary undergraduate medical reform. Nevertheless, obtaining informed agreement on the characteristics of the PBL "genus" is a challenge when the label is vulnerable to being borrowed for prestige or subversion. Many "PBL" single-subject courses within traditional curricula do not use PBL at all. Such semantic uncertainty compromises the evidence-base on the added value of problem-based versus traditional approaches and the main messages for good practice. This literature review explores what is meant by the term PBL by aiming to answer the following questions: What difficulties are inherent in the "problem-based" tag? What does the term "problem-based curriculum" imply? How has PBL been characterized and validated by focusing on its purpose? How else has PBL been characterized? How does PBL relate to problem solving? How does PBL relate to epistemological reform? In conclusion, what ground rules can be formulated for PBL? Despite much conceptual fog lingering over the PBL literature, useful ground rules can be formulated.