Abstract: Nornicotine is produced by the N-demethylation of nicotine during the post-harvest treatment of tobacco leaves.Nornicotine in tobacco serves as the precursor in synthesis of the well-characterized carcinogen N'-nitrosonornicotine (NNN), one of the tobacco-specific nitrosamines (TSNAs), and consequently damages the health of tobacco smokers and passive smokers. Studies of the mechanism of nicotine N-demethylation have, over the last decades, attracted serious attention because of its importance in plant physiological research and commerce. In this review, we discuss the history and mechanisms of nicotine Ndemethylation and the factors that affect this enzymatic reaction. The current advances in nicotine N-demethylation in tobacco provide important insights into improving the quality of tobacco and developing hazard-reduced tobacco products.