Natural visual scenes contain vast quantities of information--far more than the visual system can process in a short period of time-and spatial attention is therefore used to focus the visual system's processing resources onto a subset of the incoming visual information. Most psychological theories of attention posit a single mechanism for this focusing of attention, but recent electrophysiological studies have provided evidence that the visual system employs several separable neural mechanisms of spatial attention. This paper describes the evidence for multiple attentional mechanisms and suggests links between these neurophysiologically defined mechanisms and specific functional processes that have been proposed in psychological theories of attention.