Reflection involves active, persistent, and careful consideration of any belief or practice. It promotes understanding of underlying beliefs and application of new knowledge to new situations. Problem-Based Learning (PBL) provides the instructional mechanisms for prompting learner reflective thinking. This study examines the following: factors that students perceive as prompting reflective thinking; which factor is perceived as the most important for prompting learners' reflective thinking; whether there is a significant difference between the derived factors; and which elements or characteristics prompt reflective thinking within the derived factors. Participants were 144 sixth through eighth grade students attending three different middle schools in rural Pennsylvania. The study found that young students perceived three factors as most important in supporting their thinking: learning environment, teacher, and scaffolding tools. Reflective factors are described and implications for designing PBL that can promote reflective thinking are discussed. The findings of this study have important implications for the design of PBL environments so that they promote reflective thinking. (Contains 20 references and 3 tables.) (AEF)