Humans and other mammals perceive a vast number of volatile chemicals as having distinct odors. This ability derives from the existence of a large family of olfactory receptors that number about 350 in man and 1000 in mice. Individual odorants activate distinct combinations of olfactory receptors, generating an immense array of combinatorial receptor codes that define odorant identities. Sensory neurons in the nose express only one receptor type each and connect to the olfactory bulb in a spatially organized manner that yields a stereotyped sensory map. A secondary projection from the bulb to the cortex transforms receptor inputs, generating another, different stereotyped map that may permit the integration of inputs from combinations of receptors. Another olfactory structure in the nasal septum of animals, the vomeronasal organ, has two additional receptor families that detect pheromones and induce hormonal and behavioral responses through a different projection to the brain.