Biochemical studies and bioassays support the concept that the increased risk for cancer of the oral cavity in snuff dippers and cancer of the lung, upper aerodigestive tract, and pancreas in smokers is most likely associated with the exposure to tobacco-specific N-nitrosamines. The doses of TSNA required to induce tumors in the oral cavity in rats and tumors in the lungs of rats and hamsters are comparable to the total doses of TSNA to which a long-term snuff dipper, respectively a cigarette smoker, are exposed over 4 decades. The carcinogenic NNN and NNK are metabolized to highly reactive electrophiles which react with nucleophilic centers of DNA and with proteins. The DNA-adducts are known to cause miscoding which can activate K-ras proto-oncogenes. K-ras oncogenes have been detected in adenocarcinomas of the lung of smokers. TSNA also form globin adducts in the blood of laboratory animals as well as of snuff dippers and smokers. Such adducts are currently being evaluated for their significance as biochemical markers of exposure to TSNA. These biomarkers may also serve as indicators for the possible endogenous formation of TSNA in tobacco chewers and smokers. Additional research needs in TSNA carcinogenesis include the development of modified methods for the reduction of TSNA in tobacco and smoke, inhibition of TSNA carcinogenesis by nutrients and micro-nutrients and the testing of chemopreventive agents.