Methods for generating mutations in preselected genes of prokaryotes and eukaryotes are reviewed. Directed mutagenesis is based on mutagenic treatment of genome fragments in vitro instead of a whole genome in vivo. The cloning methods make it possible to perform directed mutagenesis even using conventional nonspecific mutagens. Methods have been elaborated for selective modification of specific sequences in cloned genes, based on the specific DNA recognition properties of restriction endonucleases and complementary poly- and oligonucleotides. Chemically synthesized oligonucleotides may be used for constructing mutant genes containing any base substitutions, small insertions and deletions.