This article presents a model of assessment development integrating student characteristics with the conceptualization, design, and implementation of standardized achievement tests. The model extends the assessment triangle proposed by the National Research Council (Pellegrino, Chudowsky, & Glaser, 2001) to consider the needs of students with disabilities and English learners on two dimensions: cognitive interaction and observation interaction. Specific steps in the test development cycle for including students with special needs are proposed following the guidelines provided by Downing (2006). Because this model of test development considers the range of student needs before test development commences, student characteristics are supported by applying the principles of universal design and appropriately aligning accommodations to address student needs. Specific guidelines for test development are presented.