Much has been said about what science content students need to learn (e.g., "Benchmarks for Science Literacy, National Science Education Standards"). Less has been said about what science content teachers need to know to teach the content students are expected to learn. This study analyzed four standards documents and assessment frameworks to identify core middle school physical science teacher content knowledge. Analysis across all four documents identified critical middle school physical science content and the relative weightings of this content. This parsimonious selection of content synthesized from these major sources provides guidelines researchers, assessment developers, and professional development providers can use in determining how to expend limited time and other resources.