Educational assessment is reasoning from observations of what students do or make in a handful of particular circumstances, to what they know or can do more broadly. Practice has changed a great deal over the past century, in response to evolving conceptions of knowledge and its acquisition, views of schooling and its purposes, and technologies for gathering and evaluating response data. Conceptions of what constitutes assessment data, how it should be interpreted, and what kind of inferences are to be drawn can differ radically under different psychological perspectives. We see greater continuity, however, when we distinguish the structure of assessment arguments from their substance. Developments here have been more in the nature of extensions, elaborations, and refinements, as they have been prompted by changes in culture and substance.