Genetic variations in the genes encoding the cytochrome P450s (CYPs) are important determinants for interindividual differences in sensitivity to drugs and environmental chemicals as well as for the pathogenesis of several human diseases. In order to standardise the nomenclature of the rapidly increasing number of alleles described, a web page was established a few years ago. Here, we describe the present status of the web page and summarise the principles used for CYP allele nomenclature.