This article exemplifies how assessment design might be grounded in theory, thereby helping to strengthen validity claims. Spanning work across multiple related projects, the article first briefly summarizes an assessment system model for the elementary and secondary levels. Next the article describes how cognitive-domain theory and principles are used in the design of a scenario-based summative assessment for argumentation in the English language arts. Finally, results from several psychometric approaches are used to evaluate propositions suggested by the domain theory, including ones related to the use of topical scenarios and learning progressions in assessment design. Although results generally supported these propositions, the work described represents only a small step in a long-term, iterative process of theory development, assessment design, and empirical tryout, which should, in principle, lead to more valid assessments that better inform teaching and learning.