Chemosensory cues stimulate male sexual arousal and behavior. The main olfactory system has an important role in attracting males to estrous females, and the vomeronasal receptors are important for activating accessory olfactory pathways that engage mating behavior in a sexually dimorphic manner. The gonadotropin releasing hormone (GnRH) neurons like the vomeronasal organ (VNO) neurons take their origin in the olfactory placode and migrate to the basal forebrain along pathfinder axons that take their origin in the developing VNO. The maturation of both systems is synchronized in time such that the early postnatal testosterone surge masculinizes the VNO neural relay en route to the medio preoptic area (MPOA). Although VNO slices and VNO receptor neurons in culture respond to volatile odors, in vivo electrophysiological recordings at the first relay in the accessory olfactory bulb (AOB) are silent until the male makes active nuzzling investigations of the female. The VNO neurons may therefore respond to volatiles that are transported into the organ on carrier peptides that themselves may play a part in receptor activation. In the context of modern molecular phylogenetic studies, it is becoming less likely that pheromones acting via the VNO have any part to play in human sexual behavior, but the possibility exists for conserved VNO genes influencing human reproduction via fertilization.