Even though the percent of adults who smoke has been reduced dramatically in the last 20 years, more than 50 million people continue to use tobacco on a daily basis. A majority of these people claim that they would like to stop smoking, but cannot. This review discusses the data suggesting that tobacco smokers are seeking nicotine. The behavioral effects elicited by nicotine and its relative reinforcing properties, as well as the development of tolerance to nicotine and the role of nicotine in the tobacco withdrawal syndrome are discussed. The data indicate that nicotine has addiction liability, but this liability is probably not equal to that of other agents such as cocaine and the opiates.