The neural architecture of a sensory system reflects two general principles: the physics of stimuli in the relevant sensory modality and the mechanisms and compromises implemented by the system to extract useful information for the organism at minimal metabolic cost. At a high level of abstraction, many of these problems are common to all sensory systems. For example, all [Page 680][Page 681]sensory systems must be able to respond to wide ranges of stimulus qualities and intensities, separate out meaningful stimuli from a background of relatively unimportant stimuli, identify similarities among stimuli so as to be able to classify them accordingly, and learn from experience. The structure of the olfactory system reflects unique functional solutions to each of these common problems. This entry will cover the ...