We have long been fascinated by the unique ability of odors to stir our emotions and to evoke long-forgotten memories, but certain odors play a much more fundamental role in that they vastly improve an organism's chances for reproductive success and survival. These odorants are called pheromones, a term commonly applied to semiochemicals that are released by one member of a species and evoke a specific reaction or reactions from members of the same species. Pheromones are known for both the specificity and the potency of their actions, which can be behavioral and/or neuroendocrinological. Pheromones can stimulate individuals to aggregate, to disperse, or to react defensively in the presence of a predator, but they are probably best known for bringing the sexes together. Some pheromones have also been found to trigger a dramatic release of pituitary hormones in several vertebrate species. Although first identified in insects, more recent studies show that sex pheromones influence the lives of a wide range of organisms, from microbes to man. The hormonally-derived sex pheromones in teleost fish, and the airborne pheromones of moths are two systems that illustrate how scientists have used these specialized chemical signals as important tools to investigate the morphology, physiology and biochemistry of olfactory-receptor systems, the mechanisms of odor-information processing in the brain, and the diverse range of behaviors and endocrinological changes associated with pheromonal communication. While our focus is on these two animal models, other examples, including mammalian pheromone systems, are also discussed.