(from the chapter) How long a period can an individual put forth maximal effort before there are negative behavioral consequences, attitudinal consequences, or both? Objective assessments of some aspects of physical fatigue have been possible since the development of the ergograph in the late 1800s by Mosso (e.g., see Mosso, 1906). The ergograph provides a record of muscular contractions, for example, when the individual repeatedly lifts a weight. Even in the case of muscular fatigue, however, it was clear to Mosso (1906) that numerous factors result in changes in patterns of fatigue, including arousal effects, time-of-day differences, motivational aspects of social facilitation, and so on. Mental or cognitive effort is clearly different from physical effort, in character, physiological activity, and time scale. The research literature on cognitive fatigue makes it clear that maximal cognitive effort can be sustained for a period longer than the few minutes that maximal physical effort can be maintained. In this chapter, I review a variety of issues that are central for considerations of cognitive fatigue in their historical and modern context. Topics to be treated include how performance aspects of cognitive fatigue are assessed, along with the myriad of task characteristics and situational characteristics that have been identified as contributing factors to cognitive fatigue. In addition, I review major theories of the mechanisms and processes that underlie the development and expression of cognitive fatigue in terms of performance and subjective fatigue. Finally, I present an integrated conceptual model of fatigue that addresses both performance and subjective fatigue. I also propose a heuristic framework of the major sources of fatigue and their probable loci of effects.