Two distinct views of the nature of clinical judgment are identified and contrasted. The dominant view that clinical judgment is a fully explicit process is compared to the relatively neglected view that tacit knowledge plays a substantial role in the clinician's mental operations. The tacit dimension of medical thinking is explored at length. The discussion suggests severe limits when applying decision analysis, expert systems, and computer-aided cost-benefit review to medicine. The goals and practices of postgraduate medical education are also examined from this perspective, as are various other implications for the clinician. The paper concludes that it is valuable to explore the nature of medical thinking in order to improve clinical practice and education. Such explorations should, however, take cognizance of the often overlooked tacit dimension of clinical judgment. Possible constraints on the medical applicability of both formal expert systems and heavily didactic instructional programs are considered.