The critical analysis of preclinical testing of anticarcinogenic and antitumor activity of biguanides presented in this paper. Experiments have been conducted using in total more than 20 models of carcinogenesis including models of spontaneous , chemically- , radiation- and virus-induced carcinogenesis, as well as carcinoigenesis induced by special fat diets and by genetic modification in rodents. Cancer preventive effect of buiguanides has been studied in relation to total tumor incidence and to 17 target organs in animals of 3 species, including 25 various strains of mice, 4 strains of rats and 1 strain of hamsters using various routs of administration and doses. In the majority of cases (86%) the exposure to biguanides leads to inhibition of carcinogenesis. In 14% of the cases inhibitory effect of the drugs was not observed, however there was no any case of stimulation of carcinogenesis by antidiabetic biguanides., Metformin suppressed tumor growth in the majority of in vitro studies conducted in 46 different cell lines originated from malignant tumors of 15 localization as well as in athymic mice with xenografts of 31 tumor lines. It was concluded that there are sufficient experimental evidences of anticarcinogenic and antitumor effects of antidiabetic biguanides revealed in a number of models of induced and spontaneous carcinogenesis.