This report summarizes the findings of a systematic review of research literature on teaching about the particle nature of matter to upper elementary students. The report is a product of the Knowledge Assets to Support the Science Instruction of Elementary Teachers (ASSET) project. ASSET is exploring a method for collecting and synthesizing pedagogical content knowledge (PCK). Literature reviews and discussions with experts will help determine what is known in the field about teaching and learning of each of the key ideas, including common patterns of student thinking and successful instructional strategies.