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 differ radically when cast under different psychological perspectives. If the structure of assessment arguments is distinguished from their substance, greater continuity is seen. Developments in this area have been more in the nature of extension, elaboration, refinement, and explication of argument structures, as they have been prompted by more radical changes in culture and substance. (Contains 13 figures and 51 references.) (Author/SLD)