(from the chapter) Fatigue can be decisively defined for physical materials because the resulting failure from fatigue is typically preceded by dislocations and deformations that can be observed with the proper tools. For humans, physical fatigue can also be indexed with physiological measurements, as first identified by Mosso (1906). In contrast, psychological fatigue, whether identified as “mental” or “cognitive” fatigue, can be revealed in a variety of ways that are often uncorrelated or only minimally correlated with one another. The most obvious domain where one would want to identify cognitive fatigue is for criteria of task performance. Direct methods of measuring cognitive fatigue often involve examining patterns of performance over time on-task (eg. Noll, 1932; Thorndike, 1926). The problem with such measures is that they are also affected by other factors, such as learning or inhibition, that are often associated with increasing time on-task, making the delineation of fatigue from other factors problematic. Another method for assessing cognitive fatigue is to ask the individuals who are performing the task to report subjective feelings of fatigue. The extant research base of measures for assessing fatigue is substantial (eg. Chalder et al., 1993; Hockey, Maule, Clough & Bdzola, 2000; Michielsen et al., 2004), and the consensus of this research is that there are several dimensions of subjective experience that are related to fatigue. However, none of these dimensions are unequivocally identifiable as fatigue, independent of other factors. For the current chapter, we will include both performance effects and subjective ratings of fatigue as indicators of cognitive fatigue.