This paper summarizes and evaluates the more important studies on fatigue as a factor in testing and other mental tasks. Several definitions of the concept of fatigue are considered and a broad definition—including subjective feelings, organic-chemical or physical change and changes in quality or quantity of work output—is accepted as the working definition for this survey. Studies are presented on fatigue from reading and other mental tasks, and the effects of variation in task and other variables upon the development of fatigue. Studies specific to testing are then considered, specifically studies on fatigue in testing— especially the question of mental fatigue as a possible factor for students taking the SATs and three achievement tests in the same day—anxiety in testing, and the effect of rest. It is concluded that at least six factors have to be considered in studying the effects of fatigue on mental tasks: interference, distraction, homogeneity of task, transfer of fatigue from one task to another, anxiety in testing, and rest. Three conclusions are drawn from the discussed studies: 1) sufficient evidence exists in the literature to discount any likelihood of physiological consequences to the development of fatigue during a candidate's taking the College Board SAT or Achievement Tests; 2) the decline in feeling-tone experienced by an individual is often symptomatic of developing fatigue, but this decline does not necessarily indicate a decline in the quantity or quality or work output; and 3) the amount of fatigue that develops as a result of mental work is related to the individual's conception of, and attitude and motivation toward, the task being performed. It is suggested that future research study in more detail the effects on male and female students taking the SATs and achievement tests at different administrations and on the morning and afternoon of the same day. These studies should take into consideration such moderator variables as the subjective feelings of fatigue for each candidate both prior to and subsequent to taking the tests; the candidates' motivation for taking the tests; the candidates' pre- and post-test perception of the task; and the pretest anxiety level of each candidate.