Problem-based learning (PBL) is an instructional method where student learning occurs in the context of solving an authentic problem. PBL was initially developed out of an instructional need to help medical school students learn their basic sciences knowledge in a way that would be more lasting while helping to develop clinical skills simultaneously. Although PBL addresses this specific need, it is also based in sound educational theories and paradigms. The author addresses those theoretical foundations of PBL, which, in turn, help readers to understand why PBL can be effective as well as enable them to diagnose and improve PBL applications when things are not going quite as planned.