Current information about action, function and polymorphism of Cytochrome P450 enzymes is presented in the paper. The different phenotypes (EM, UM, PM), and the meaning of phenotypes for evaluation of genetically determinated reactions to administrated drugs are characterized. The kind of phenotype can be a genetic marker of many diseases. Phenotyping methods are described (Sparteine Test, Debrisoquine Test). The latest information on molecular biology achievements related to the problem of genotyping is also given.