We describe major discoveries and developments in vigilance research. Vigilance tasks have typically been viewed as undemanding assignments requiring little mental effort. The vigilance decrement function has also been considered to result from a decline in arousal brought about by understimulation. Recent research in vigilance is reviewed in four areas: studies of task type, perceived mental workload during vigilance, neural measures of resource demand in vigilance, and studies of task-induced stress. Experiments comparing successive and simultaneous vigilance tasks support an attentional resource theory of vigilance. Subjective reports also show that the workload of vigilance is high and sensitive to factors that increase processing demands. Neuroimaging studies using transcranial Doppler sonography provide strong, independent evidence for resource changes linked to performance decrement in vigilance tasks. Finally, physiological and subjective reports confirm that vigilance tasks reduce task engagement and increase distress and that these changes rise with increased task difficulty. Converging evidence using behavioral, neural, and subjective measures shows that vigilance requires hard mental work and is stressful. This research applies to most human-machine systems that require human monitoring, particularly those involving automated subsystems.