Odours play a critical role in the behaviour and physiology of many species. For mice and probably many other species, including humans, an individual's olfactory identity (its odourtype) is coded in part by a pattern of volatile compounds that is regulated by genes in the major histocompatibility complex, a string of linked genes that is intimately involved in immune function. The mouse olfactory system is exquisitely sensitive to minute variations in odourtypes. Layered within these chemical signals of individuality is information on the age and health status of the mouse. In the case of age, it appears that information is coded based on a pattern of volatile metabolites; we do not know how a mouse detects, for example, the presence of a viral infection in volatiles from an infected mouse. This chemical information serves to regulate mate choice and other aspects of social behaviour.