Our studies of mental fatigue utilize tasks entailing continuous (50-60 min) demands for memory, anticipation and judgment processes, under conditions in which active eye movement patterns are required. Our review of these studies will emphasize two key aspects. One is that an important component of fatigue is the increased incidence of momentary lapses of attention or alertness, in the context of otherwise satisfactory performance. Such lapses may be infrequent but, if occurring at inopportune times, can have catastrophic consequences. Our research has included efforts to identify EEG and ERP signs during such lapses, and as well as premonitory signs of impending lapses. Of particular promise appears to be the appearance of transient EEG features which are customarily thought to accompany sleep onset under eyes-closed conditions (Santamaria and Chiappa, 1987). We show that such transients can be detected in the waking EEG even with eyes open and during active task performance, and that some are associated with impaired performance. The second key aspect is that oculomotor activities are a sensitive convergent measure of mental fatigue. The majority of performance lapses during continuous tasks are within context of some demonstrable oculomotor indicator of fatigue. Since the oculomotor activities are continuous measures, they serve to identify momentary lapses on a finer-grained time scale than is afforded by typical task performance-based measures. Their utility within this context is reinforced by our findings that impaired oculomotor performance is accompanied by local EEG signs of fatigue (preponderance of energy at slow frequencies, increased incidence of transients).