This report describes efforts by a group of science teachers, teacher educators, researchers, and content specialists to conceptualize, develop, and pilot practice-based assessment items designed to measure elementary science teachers' content knowledge for teaching (CKT). The report documents the framework used to specify the content-specific teaching practices and instructional tools that are critical to the work that elementary science teachers engage in with students, curriculum, and instruction. Drawing on this framework, the report details the development process for practice-based assessment items designed to measure CKT elementary science in three content areas: (a) structure and properties of matter, (b) ecosystems, and (c) Earth's place in the universe. These practice-based assessment items address the various content challenges elementary science teachers face in their work and were designed to be used as a foundation for building large-scale assessments of elementary science teachers' CKT science. This report presents initial validity evidence examining the practice-based CKT science item characteristics using results from online administrations of these items to 250 upper elementary science teachers. Findings reveal that the majority of these new assessment items capture variability in elementary science teachers' performance and that a large proportion of the items differentiate moderately well, supporting a beginning proof-of-concept for the conceptualization and design of these practice-based CKT elementary science assessment items.