There is a broad body of research reporting key understandings and foundational reasoning abilities (e.g., Carlson & Rasmussen, 2008) students need to understand and be successful in calculus. Some projects (e.g., Project Pathways at Arizona State University) have leveraged these findings to inform the development of curriculum (e.g., Pathways Precalculus) and mathematics assessment tools such as the Precalculus Concept Assessment (PCA) (Carlson, Oehrtman and Engelke, 2005), Calculus Concept Readiness (CCR) (Carlson, Madison and West, in press) and the Algebra, Precalculus Concept Readiness (APCR) (Madison, Carlson, Oehrtman & Tallman, accepted) exams. In this session we will present a broad taxonomy of key understandings and reasoning abilities that guided the development of these exams, and are needed for learning calculus. We will illustrate how this taxonomy of key ideas and reasoning abilities informed the initial development and adaptation of research based placement tests and curriculum. Comparison data that reveals qualitative differences in student learning in precalculus will be shared along with data that reveals how assessment results can be used to shape and adapt curriculum.