The 1990s became the decade that olfactory neuroscience showed extraordinary development. Now we have much better understanding of how we distinguish various odors and how the signaling pathways are in the brain. We also know that the olfactory system is not a single system but it is a group of systems in the nasal cavity that respond to chemical compounds. It has been believed for decades that pheromones are received at the vomeronasal organ and the signaling pathway from there reaches to hypothalamus and activates GnRH neurons, however, recent studies have determined that the pathway that reaches to GnRH neurons starts from the main olfactory epithelium. There are pheromones that have receptors in the accessory olfactory system releasing behaviors. The responses to odors/pheromones are not always the same but they change at the sensory neuron level and these changes are regulated by hormones. It is a chemical signaling process.