This chapter considers the need for a better understanding of complex human cognition in the design of interactive visual interfaces by surveying the availability of pertinent cognitive models and applicable research in the behavioral sciences, and finds that there are no operational models or useful precedent to effectively guide the design of visually enabled interfaces. Further, this chapter explores the impact of individual differences, and in particular, inherent differences such as personality factors, on complex cognition. Lastly, it outlines how knowledge of human individuality, coupled with what is known about complex cognition, is being used to develop predictive measures for interface interaction design and evaluation, a research program known as the Personal Equation of Interaction. …