The biological basis of human attention has been greatly advanced through the use of nonhuman primates as model organisms. Humans are old world primates, and our visual system is highly similar to other old world primates such as macaque monkeys. Monkeys can be trained on visual and eye movement tasks such as attending to peripheral stimuli while holding their eyes still. This makes them ideal subjects for studying neurophysiological correlates of attention. Over the past few decades, such studies have revealed computational mechanisms through which attention modulates visual responses. At the level of individual cortical neurons, attention can act to increase the gain and reduce the variability of responses to attended visual stimuli. These results set the stage for the development of a biologically plausible standard model of sensory attention. By applying the principles of signal detection theory, this model can link physiological effects of attention to psychophysical performance. The general approach of quantitatively accounting for behavior based on neuronal activity can be used to refine and elaborate the standard model.