Problem-based learning (PBL) is an instructional model that assumes the centrality of problems to learning. Research on PBL has focused on student learning, student roles, tutor roles, problem design, and technology use (Hung, Jonassen, & Liu, 2008), but little attention in the PBL literature has been paid to the nature of the problems that provide the focus for PBL. In this paper, we articulate a model for evaluating problem difficulty. Problem difficulty is define in terms of complexity, including breadth of knowledge, attainment level, intricacy of procedures, relational complexity, and problem structuredness including intransparency, heterogeneity of interpretations, interdisciplinarity, dynamicity, or competing alternatives. Based on these characteristics, we identify four classes of problems and then describe three different kinds of problems: decision-making, diagnosis-solution, and policy problems. We then examine the amenability of these classes and problem types as foci for PBL curricula. Finally, we challenge PBL researchers and designers to consider the issue of problem difficulty in articulating PBL curricula.