The major advantage of understanding the population genetics of xenobiotic metabolising cytochromes P450 is the ability to predict the efficacy and side effects of a given drug in a population with a particular frequency distribution of the polymorphic alleles as well as in an individual with a particular genotype. This opens the possibility to design and select drugs that are the most efficient and have the lowest risk of unwanted side effects in our population and in a particular patient. The studies of genetic polymorphism influencing carcinogen activation and detoxification contributed to our understanding of genetic susceptibility to environmental carcinogenesis in Slovenian population and are important for planning efficient cancer prevention.