Background 1 2 3 4 6 7 8 9 10 11 12 13 14 1A 15 16 17 18 19 Figure 1 Comparison of growth rates of PBL and biomedical knowledge. (A), Relative growth of interest in PBL versus lecture-based teaching based on PubMed keyword frequencies between 1975 and 2000. The Title fields of the journal database were searched each year for the strings "problem-based learning" (open squares) or "lecture" (open diamonds); the retrieved items were then scrutinized to determine those dealing with the subject of teaching style (e.g., eponymous "lectures" were excluded). (B), Growth rates of journal articles dealing with subject matter relating to science, medicine and education. The strings "gene" (solid diamonds), "clinical" (open squares) and "medical education" (solid triangles) are shown here, illustrative of the frequencies of many other keywords searched.  20 21 Discussion What is knowledge – anyone know? 22 x  y  23 24 1B 25 26 27 28 29 The lure of the non-expert 30 31 32 33 34 38 39 40 41 42 43 44 45 Things have changed Efficiency can be calculated by dividing (productivity) by (time and effort). What do we mean in this context by productivity? A half-century ago, the only responsibility of a medical school was to produce clinicians to serve the local community; today, however, teaching activities incorporate postgraduate specialist education, continuing medical education, professional and career development, public and patient awareness programmes, education-related research, conference and workshop organization, national and international collaborative initiatives, professional accreditation and audit activities, development of electronic teaching resources, and so on. Hence, a modern faculty's teaching productivity is not able to be gauged exclusively (or even predominantly) by the number and quality of its outgoing medical graduates, but rather must be judged by the sum total of its useful educational output. 46 47 49 50 52 53 54 56 57 62 From words to actions Solutions lie in compromise. Such change is painful because it involves the abandonment of ideals formerly attainable; the vision of a one-size-fits-all medical school becomes no longer practical, and ever more difficult decisions will be needed as to what style(s) of graduate is most urgent for a faculty to produce. This process of curricular differentiation has started, but the pace is set to quicken as medical markets emerge and diverge, and as competition for faculty survival sharpens. To what extent, though, should these divisive educational decisions be made by markets, faculties, students, patients or governments? Contrary to popular thought, there will remain a strong need – and possibly an enlarging one – for a subset of highly-trained medical graduates from a knowledge-intensive learning environment who are capable of assimilating the complexities of science, informatics, humanities and logistics that comprise modern medicine. Since the proportion of individuals and faculties suitable for this leadership mission looks set to decline, however, a larger number will need to accept the equally daunting compromise of skills prioritization. Teachers cannot teach without students, but students can learn without teachers. This belated insight has transformed the role of teachers into that of learning facilitators, akin to a culture of "thinking apprenticeship". Paradoxically, in an age when even complex skills such as landing aircraft are learned using robotic simulators, the trend in medical education has switched back to labor-intensive small-group teaching under the guise of PBL. This at first seems all the more curious given the unprecedented availability of alternative technologies for teaching clinical reasoning, the increasing importance of an adequate knowledge base in an ever more sophisticated professional environment, the growing pressures on faculties to use limited fiscal resources in the most cost-effective manner, and the novel opportunities for commercializing educational activities and products via the development of software and web-based resources. 63 Summary The knowledge explosion of the last two decades has been accompanied by a decreasing reliance on didactic teaching. This educational paradigm shift has been led by widespread embracement of PBL, the original rationale of which was to improve students' ability to reason and communicate. In recent years, however, PBL has grown more rapidly in apparent response to information overload in medical school curricula, and may thus be viewed as a symptom of the problem of biomedical knowledge expansion. The challenge of defining the right balance between what is taught, what is learned, and what remains unlearned will not disappear. Although few knowledge deficits have been detected in today's PBL-educated students, a decreasing concern with the adequacy of the professional knowledge base could yet erode the future credibility of the medical profession. By continuing to rely on popular PBL escape clauses such as 'self-directed learning' and 'information management', today's medical educators risk losing sight of this longterm threat. The era of active learning began thousands of years ago with the first apprentice. We now live in a new era with new challenges, one of which is exponential information expansion. PBL provides one way for faculty and students to cope with this challenge, but sidesteps deeper issues relating to the widening core of essential professional knowledge. Innovative curricular experiments using educational strategies complementary to PBL would therefore appear timely. Abbreviations PBL, problem-based learning Competing interests None declared. Pre-publication history The pre-publication history for this paper can be accessed here: