Visual perception requires selective filtering. The process of selecting a portion of the visual input according to its location is described as spatial attention. Spatial attention has been measured with a wide variety of experimental techniques, including spatial cuing, spatial probes, distractor interference, ERP, and SSVEP. The results show that spatial attention sometimes takes the form of a gradient, with strong facilitation of processing within a central region and less facilitation and perhaps even inhibition in the surround. The positioning of the attentional gradient is controlled in part by a bottom-up system that directs attention to locations that differ from surrounding locations in basic features. There is also top-down direction of attention, which favors locations with features matching a defined target. A variety of different experiments have demonstrated that attention can be allocated to a particular location in the visual field, but another set of experiments show that attention can be allocated to a visual object, and that attention that is directed to one part of an object can spread to other parts of the same object. It is difficult to determine whether spatial attention and object-based attention are controlled by the same system or by separate systems. Determining the boundaries between different attentional systems should become easier with the use of ERP data to provide precise timing information about attentional processes, and fMRI to localize the brain regions controlling attention and to measure attentional modulation of perceptual processing activity.