Immediately following an event at a peripheral location there is facilitation for the processing of other stimuli near that location. This is said to reflect a reflexive shift of attention towards the source of stimulation. After attention is removed from such a peripheral location, there is then delayed responding to stimuli subsequently displayed there. This inhibitory aftereffect, first described in 1984 and later labeled 'inhibition of return (IOR)', encourages orienting towards novel locations and hence might facilitate foraging and other search behaviors. Since its relatively recent discovery, IOR has been the subject of intensive investigation, from many angles and with a wide variety of approaches. After describing the seminal contribution of Posner and Cohen ('Who'), this review will discuss what causes IOR and, once initiated, what effects IOR has on subsequent processing ('What'). The time course ('When') and spatial distribution ('Where') of IOR, and what is known about IOR's neural implementation ('How') and functional significance ('Why') are also discussed.