Richard E. Snow conceived of an individuals' performance as the result of a transaction between her aptitudes and the particular characteristics of the situation in which that performance occurs over time. By aptitudes he meant the cognitive and motivational resources that an individual brings to the situation. By situation he meant the characteristics of a particular environment that afforded or impeded that assisted or constrained certain courses of goal-related action. When Snow applied his ideas to achievement testing situations, he recognized that test performance resulted from a students background and intellectual history, as well as the cognitive and motivational resources the student cobbled together to respond to a series of situation-embedded test tasks (e.g., multiple-choice items or performance assessments). In essence, Snow's idea was that individuals achievement test performance depended not just on their knowledge and abilities, but rather on a full spectrum of interrelated, situation-relevant cognitive and motivational resources that transacted with the affordance, constraint, and demand structures of the testing tasks themselves. From this reasoning and from empirical findings he concluded that a new multivariate approach to validating interpretations of achievement test scores was needed. This document sets forth in more detail Snow's new aptitude theory as it applies to multivariate test validity and then described the design of the High School Study that was conducted as a step in exploring his ideas about validity. (Contains 3 tables and 34 references.) (Author/SLD)