Odor transduction and quality coding involves a cascade of events that occur at the level of the olfactory epithelium and olfactory bulbs. Odorants bind to one or a few specific olfactory receptors located in the cilia of olfactory neurons. These olfactory receptor proteins make up the largest gene family discovered and are diverse between and within species. The change of chemical signals to neural signals in the olfactory neurons involves G-coupled proteins and the cascade of second messenger pathways that open ion channels to depolarize the cell and trigger a series of action potentials carried along the receptor cell axon resulting in release of glutamate at synapses with mitral cells within the olfactory bulb. These neural signals in the olfactory bulb produce unique odor maps that play an important role in our ability to detect and discriminate thousands of different odorants. The olfactory neurons are replaced throughout life from a population of slowly dividing basal cells within the epithelium. Disease, infection, injury or aging can interfere with neuronal cell replacement as well as transduction and coding processes, resulting in impairment and distortions of olfactory performance.