The vomeronasal organ (VNO) and accessory olfactory system (AOS) are present in most terrestrial vertebrates except birds and higher primates. The receptor neurons of the AOS are sequestered inside the VNO, away from the main airflow to the main olfactory receptor neurons. Mechanisms of stimulus access to the sensory neurons vary across species but in most cases there is a system for delivering stimuli faster than would be possible by diffusion. Vomeronasal (VN) receptor neurons typically lack cilia, the site of most of the transduction apparatus in the main olfactory receptors. The VN receptor neurons have a restricted but privileged pathway to the areas of the brain concerned with reproduction and social behavior. In contrast, the main olfactory neurons have a broad pathway to wide areas of the brain, including the neocortex. Experiments where the VNOs or other parts of the accessory olfactory pathway were ablated indicate that the system is important in many behavioral and physiological responses to pheromones (chemical signals carrying information about gender or reproductive or dominance status), some of which may be proteins. VN sensory neurons respond to both volatile and non-volatile stimuli. There is no evidence in the vertebrate AOS for the extreme sensitivity or selectivity characteristic of insect pheromone detectors, but this has not been adequately tested. There is some evidence for learning, possibly by synaptic modification at the second-order neuron level. Social and reproductive cues stimulating the AOS often elicit an intracerebral release of LHRH--which may act at receptors different from those of the pituitary to facilitate behavior. Whether the LHRH release is necessary for AOS-mediated behavioral response is not yet clear.