This conclusion presents some closing thoughts on the concepts covered in the preceding chapters of this book. The book describes the various methods employed for the syntheses of tobacco-specific N-nitrosamines (TSNA) and their metabolites and adducts. It presents an overview on the occurrence of oral cancer as it relates to tobacco use. The book discusses the metabolism of the highly carcinogenic NNK in laboratory animals and humans as well as the involvement of enzymes in the bioactivation of this TSNA. It also presents an overview of the endogenous formation of carcinogenic N-nitrosamines, especially as it relates to TSNA in tobacco smokers, snuff dippers, and chewers of tobacco and of betel quid mixtures with tobacco. Whereas it has been clearly demonstrated that TSNA are formed during the chewing of tobacco, the question about endogenous formation of NNK from nicotine in smokers is still not fully elucidated.