Problem-based learning (PBL) is finding a place in more and more secondary school science programs as teachers discover its power to engage students and develop critical-thinking skills. In the best PBL scenarios, constructivist learning is driven by challenging, open-ended problems: students work in collaborative groups and teachers become facilitators. When students work together to solve authentic, meaningful problems, they appreciate the priority of using evidence to make decisions as they critically evaluate possible solutions. Throughout this issue of TST, readers will find many examples of possible problems to use for classroom inquiry and student investigations.