This project is advancing undergraduate STEM education through the improvement of ways in which science majors, who anticipate becoming science teachers at the secondary level (middle school and high school), are prepared to engage in formative assessment reasoning that is grounded in evidence of student learning and understanding. Traditional secondary level school assessment methods stress grading with too little emphasis on the role of assessment in fostering student learning. This encourages rote and superficial learning with teaching practices tending to emphasize completion of work over the quality of student understanding. This project seeks to revolutionize teachers' assessment practices as crucial to advancing educational reform. The work especially focuses on formative assessment, the type of assessment that holds the most direct promise for enhancing student learning in classrooms. The prospective teachers in this project are undergraduate science majors preparing to become middle school and high school science teachers through a teacher preparation program in the College of Science at the University of Arizona. The project is being accomplished in four phases: (1) development of assessment probes to diagnose prospective science teachers' formative assessment reasoning skills at different points in the science teacher preparation program, (2) creation of two "maps," one that characterizes the progression of prospective teachers' reasoning throughout the program and a second map of expert teacher standards for formative assessment reasoning, (3) development and implementation of instructional activities designed to improve the alignment between prospective teachers' reasoning progression and the expert teacher reasoning map, and (4) analysis of the learning outcomes associated with the implemented instructional activities followed by revision of the activities. The products created in this project - assessment probes and instructional activities - will be sustainable and directly incorporated in the daily instructional practices in the teacher preparation program at the University of Arizona. All of the products and research findings of this project will be fully available for other science teacher educators for use in teacher preparation programs across the nation.