Because perceiving is more than simply detecting the information provided by the information reaching the retina or the tympanic membrane, Chap. 8 emphasizes the role of attention in perception. After describing change blindness and attentional blindness, experimental conditions are reported showing that it is possible to prepare the perception of stimuli in advance with some spatial cues, or at a given moment in time. Then, because there is always present, within our reach, a multitude of information provided by our sensory receptors, there is a need for selecting specific information, which is one of the main functions of the attentional processes. The study of visual selectivity and auditory selectivity are both described. The last part of the chapter is dedicated to a task thatis required in everyday life, i.e., to search actively for something in a set of stimuli. This task is called visual search.