Olfaction is the sense of smell, which is mediated by specialized sensory neurons in the nasal cavity. Classically, the olfactory system is divided into two anatomically independent olfactory systems, the main olfactory system and the accessory olfactory system. Evidence from rodents indicates the presence of other spatially segregated subpopulations of sensory neurons within the mammalian nose. These subpopulations project to different brain areas and process chemosensory information ranging from location of prey and predators, the sexual, hormonal, and reproductive as well as health state of (mating) partners, and care of offspring to the level of aggression in rivals. These subpopulations of sensory neurons use different signaling cascades for the transduction of information from chemosensory molecules into electrical membrane signals. Considerable progress has been achieved in the comprehension of how various olfactory cues are detected and can influence social behavior. But, in many cases questions remain regarding the mechanisms in the brain by which olfactory stimuli influence physiology and behavior in specific brain regions and regarding the individual neuronal circuits and neurons that mediate these effects. The current knowledge on the behavioral, physiological, and molecular aspects of odor detection in mammals including humans is presented in this chapter.