The purpose of this article is to present the application of the universal design model in the broad field of psychological assessment for education, covering the area of creating diagnostic tools as well as designing the process of assessment. A universally designed diagnostic tool enables fair and valid assessments of a wide range of users, including individuals with special educational needs. It also helps prevent bias in test scoring and interpretation and, at the same time, enables fairness in test use. The authors present the synthesis of the guidelines concerning the design of universal diagnostic tools and the diagnostic process itself at four key levels: conceptual, formal, test administration, and interpretative. As an example of a universally designed tool, the Comprehensive Analysis of Cognitive Processes (KAPP) has been mentioned. Discussion includes the benefits and limitations of applying the idea of universal design to psychological assessment with an emphasis on test development.