Jacobson's organ of the vomeronasal system is found in every order of mammals with the possible exception of Cetacea. The equivocal evidence claiming a vestigial or absent organ in humans is reviewed. Based upon anatomical considerations, the sensory epithelium of Jacobson's organ is one of five possible sensory components within the nasal cavity. Many methods designed to test the role of olfaction (sensu strictu) in physiology and behavior do not discriminate among the possible systems. Therefore, erroneous conclusions may have been drawn from the results of intervention experiments. The central neuroanatomical projections of the vomeronasal and olfactory systems are different and relatively independent of each other. The vomeronasal system reciprocally communicates with central areas concerned with reproductive events. On the other hand, the olfactory system may subserve individual maintenance tasks (e.g., feeding). As a periscope from the diencephalon, the vomeronasal system may monitor exogenous hormones, "pheromones".