The capacity to sustain attention at an efficient level deteriorates over time in discrimination and monitoring tasks. This "vigilance decrement" results from a decrement in perceptual sensitivity only if (i) target discrimination loads memory and (ii) stimulus events occur rapidly; otherwise, the decrement reflects temporal changes in response criteria. These results provide a basis for distinguishing between the perceptual and response processes underlying the vigilance decrement that may be generalized across a range of tasks.