One of the underlying assumptions of Problem-Based Learning (PBL) is that it motivates students to study. For instance, feeling of being in charge of one’s own learning, working in small groups under the limited guidance of a teacher, working on real-life problems and puzzles are all factors that are expected to make learning more interesting. To date, there are, however, only a very little number of studies that investigate whether PBL has indeed a motivating effect on student learning. In this chapter, the results of two recently conducted studies are summarized that measured student motivation by means of students’ levels of situational interest in the PBL classroom. The first study examined how interest developed during a PBL lesson of one day. For instance, it was measured whether presenting a problem increases student interest, and what happens to their levels of interest during self-study and reporting of findings at the end of the day. The second study explored in how far facilitators impact student interest in PBL. To that end, three facilitator characteristics were measured: (1) subject-matter expertise (i.e. the knowledge a facilitator has); (2) social congruence (i.e. how well a facilitator is able to “tune in” to the students); and (3) cognitive congruence (i.e. a facilitator’s ability to encourage and enable student learning). It was then investigated which of these factors has the largest impact on student interest. The results of both studies provide more insights in the intricate mechanisms responsible to motivate students to study in PBL. The finding and implications of these studies for PBL research and practice are discussed.