This document discusses the physiology of smell. It describes the anatomy of the olfactory system, including the olfactory epithelium containing olfactory receptor neurons with cilia that project into mucus and are regenerated every 60 days. It also explains olfactory transduction, noting that there are over 1000 receptors that can detect over 10000 different odors through G protein-coupled receptors and second messenger systems. Factors that influence smell like stimulus type, threshold, intensity and adaptation are also outlined, as well as pathologies involving reduced, absent, distorted or perverted smell.