The initial step in olfactory discrimination involves the interaction of odorous ligands with specific receptors on the surface of olfactory sensory neurons. The foundation for a molecular understanding of odor recognition in vertebrates was provided by the identification of a family of genes encoding putative odorant receptors, by Buck & Axel in 1991. Odorant receptor (OR) genes from the largest gene family in the vertebrate genome. This review summarizes progress over the past seven years. Major new insights are: Olfaction is accomplished in vertebrates by a very large number of receptors; olfactory sensory neurons express a small subset of the OR repertoire; in rat and mouse, axons of neurons expressing the same OR converge onto defined glomeruli in the olfactory bulb.