Examinees' thinking processes have become an increasingly important concern in testing. The responses processes aspect is a major component of validity, and contemporary tests increasingly involve specifications about the cognitive complexity of examinees' response processes. Yet, empirical research findings on examinees' cognitive processes are not often available either to provide evidence for validity or to guide the design or selection of items. In this article, studies and developments from the author's research program are presented to illustrate how empirical studies on examinees' thinking processes can impact item and test design.