Fatigue is a serious problem. The rising amount of people suffering from mental overload urges to gain more knowledge on the mechanisms underlying fatigue. This knowledge is essential in order to be able to cope effectively with fatigue, and more important to be able to prevent humans from becoming, for instance, work disabled due to mental overload. At the subjective level, fatigue manifests itself in reduced motivation and in negative emotions. With respect to performance, the picture is less clear. In general, fatigue is related to a deterioration of cognitive functions. An important question, however, is which cognitive processes underlie the performance changes observed under conditions of mental fatigue elicited by prolonged task performance. Recent research suggests that when people become fatigued, the flexibility that characterises normal human behaviour seems to disappear: behaviour becomes less dynamic and more rigid. Cognitive control mechanisms lie at the heart of dynamic behaviour, and play an important role in novel and complex task situations, conditions under which fatigued humans have most pronounced performance difficulties. We found, for example, that with increasing mental fatigue humans failed to use available information to prepare and guide performance in order to function efficiently and to cope with complex situations. These results appear to bear out that the locus of behavioural control shifts from internal to external factors, under conditions of fatigue.