The present article elaborates on cognitive effects of problem-based learning put forward by Schmidt, De Volder, De Grave, Moust & Patel (1989) and Norman & Schmidt (1992). Its purpose is to discuss, in some detail, the theoretical premises of this approach to learning and instruction. It is argued that problem-based learning, above all, promotes the activation of prior knowledge and its elaboration. Evidence is reviewed demonstrating that these processes actually occur in small-group tutorials and that the processing of new information is indeed facilitated by discussion of a relevant problem. These effects must be attributed to a reorganization taking place in the knowledge structures of students as a result of problem-oriented study. In addition, a cognitive process called epistemic curiosity (or intrinsic interest) is enabled. Some directions for further research are outlined. The contribution starts, however, with a discussion of the philosophical and pedagogical roots of problem-based learning.