N‑Nitrosonornicotine (NNN) is a nitrosamine compound. It is produced by nitrosation of nicotine during the curing, aging, processing, and smoking of tobacco. About half of the NNN originates in the unburnt tobacco, whereas the remainder is formed during burning. NNN is reasonably anticipated to be a human carcinogen based on sufficient evidence of carcinogenicity from studies in experimental animals. NNN induces deleterious mutations in oncogenes and tumor suppression genes by forming DNA adducts, which could be considered as tumor initiation. Meanwhile, the binding of NNN to the nicotinic acetylcholine receptor promotes tumor growth by enhancing and deregulating cell proliferation, survival, migration, and invasion, thereby creating a microenvironment for tumor growth.