This lecture will summarize recent insights into the cellular and molecular mechanisms underlying pheromone sensing by the mammalian olfactory system. Previously unknown families of pheromonal signals have been identified, which are expanding the traditional definition of a pheromone. Electrophysiological and large-scale imaging studies in gene-targeted mice have revealed an unexpected diversity of chemosensory systems and their underlying cellular and molecular mechanisms. These studies have identified specific receptors and second messenger-operated ion channels of various gene families as key elements in pheromone detection. Targeted mutation of some of these genes leads to dramatic changes in social behavior. Ongoing experiments are revealing new insights into the mechanisms that underlie the sensory evaluation of gene composition of a potential mating partner by the olfactory system.