This paper discusses some of the most important conceptual, practical, and policy issues surrounding the what and how of assessment in K–12 education, especially in the context of current federal and state initiatives related to the Race to the Top. The general argument developed in this paper is that the learning sciences have made great strides in understanding the nature of learning and knowing, in ways that can and must inform the development of systems of assessments and the selection and use of models for measuring the status and growth of academic achievement in core areas of the curriculum. In developing this argument, the paper discusses a set of interconnected and critical issues regarding the nature and function assessment in the larger educational enterprise.