Educational test theory consists of statistical and methodological tools to support inferences about examinees' knowledge, skills, and accomplishments. The evolution of test theory has been shaped by the nature of users' inferences which, until recently, have been framed almost exclusively in terms of trait and behavioral psychology. Progress in the methodology of test theory enabled users to extend the range of inference, sharpen their logic, and ground their interpretations more solidly within these psychological paradigms. In particular, the focus remained on students' overall tendency to perform in prespecified ways in prespecified domains of tasks; for example, to make correct answers to mixed-number subtraction problems. Developments in cognitive and developmental psychology broaden the range of desired inferences, especially to conjectures about the nature and acquisition of students' knowledge. Commensurately broader ranges of data-types and student models are entertained. The same underlying principles of inference that led to standard test theory can be applied to support inference in this broader universe of discourse. Familiar models and methods--sometimes extended, sometimes reinterpreted, sometimes applied to problems wholly different from those to which they were first devised--can play a useful role to this end. Contains three tables and seven figures. (Author)