This is an Early-Stage Design and Development collaborative effort submitted to the assessment strand of the Discovery Research PreK-12 (DRK-12) Program. Its fundamental purpose is to examine and gather initial validity evidence for assessments designed to measure and build Kindergarten-fifth grade science teachers' content knowledge for teaching (CKT) about matter and its interactions in teacher education settings. The selection of this topic will facilitate the development of a proof-of-concept to determine if and how CKT assessments can be developed and used to measure and build elementary teachers' CKT. Also, it will facilitate rapid and targeted refinement of an evidence-centered design process that could be applied to other science topics. Plans are to integrate CKT assessments and related resources into teacher education courses to support the ability of teachers to apply their content knowledge to the work of teaching and learning science. The project will combine efforts from prior projects and engage in foundational research to examine the nature of teachers' CKT and to build theories and hypotheses about the productive use and design of CKT assessment materials to support formative and summative uses. Likewise, the project will create a set of descriptive cases highlighting the use of these tools. Understanding how CKT science assessments can be leveraged as summative tools to evaluate current efforts, and as formative tools to build elementary teachers' specialized, practice-based knowledge will be the central foci of this effort. <br/><br/>The main research questions will be: (1) What is the nature of elementary science teachers' CKT about matter and its interactions?; and (2) How can the development of prospective elementary teachers' CKT be supported within teacher education? To address the research questions, the study will employ a mixed-methods, design-based research approach to gather various sources of validity evidence to support the formative and summative use of the CKT instrument, instructional tasks, and supporting materials. The project will be organized around two main research and development strands. Strand One will build an empirically grounded understanding of the nature of elementary teachers' CKT. Strand Two will focus on developing and studying how CKT instructional tasks can be used formatively within teacher education settings to build elementary teachers' CKT. In addition, the project will refine a conceptual framework that identifies the science-specific teaching practices that comprise the work of teaching science. This will be used as well to assess the CKT that teachers leverage when recognizing, understanding, and responding to the content-intensive practices that they engage in as they teach science. To that end, the study will build on two existing frameworks from prior NSF-funded work. The first was originally developed to create CKT assessments for elementary and middle school teachers in English Language Arts and mathematics. The second focuses on the content challenges that novice elementary science teachers face. It is organized by the instructional tools and practices that elementary science teachers use, such as scientific models and explanations. These instructional practices cut across those addressed in the Next Generation Science Standards' (NGSS; Lead States, 2013) disciplinary strands. The main project's outcomes will be knowledge that builds and refines theories about the nature of elementary teachers' CKT, and how CKT elementary science assessment materials can be designed productively for formative and summative purposes. The project will also result in the development of a suite of valid and reliable assessments that afford interpretations on CKT matter proficiency and can be used to monitor elementary teachers learning. An external advisory board will provide formative and summative feedback on the project's activities and progress.<br/><br/>The DRK-12 Program seeks to significantly enhance the learning and teaching of science, technology, engineering and mathematics (STEM) by preK-12 students and teachers, through research and development of innovative resources, models and tools. Projects in the program build on fundamental research in STEM education and prior research and development efforts that provide theoretical and empirical justification for proposed projects.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.