This chapter is about what constitutes high quality and valid assessment from a learning sciences perspective. It argues that assessment is a process of reasoning from evidence based upon three critical components that must work together: (1) theories and research about content-based cognition that identify the knowledge and skills of interest; (2) tasks and observations that can provide evidence and information about student mastery, and (3) qualitative and quantitative interpretive techniques that capture important variation in knowledge and skill among students being assessed. Assessment design involves application of principled processes focused on collection of relevant evidence and its use in argumentation regarding validity of the inferences to be drawn about meaning and use of the results. Assessment development should be seen as a major form of conceptual research within the learning sciences. It requires multidisciplinary collaborations that include learning sciences researchers in academic disciplines working together with instructional and measurement experts.