Addicted smokers experience nicotine withdrawal anytime they go too long without smoking. Withdrawal presents as a continuum of symptoms of escalating severity described by smokers as “wanting,” then “craving,” and eventually “needing” to smoke. These may be followed by irritability, impatience, moodiness, difficulty concentrating, restlessness, and sleep disturbances. This spectrum of intensifying withdrawal symptoms creates a compulsion to smoke that makes quitting difficult. The compulsion to smoke is the core feature of nicotine addiction accounting for its clinical course, physiological characteristics, prognosis, and behavioral manifestations. A compulsion can develop quickly, having been experienced by one third of youth who have smoked only 3 or 4 cigarettes. Its physiologic basis is evident in neurophysiological measures and its recurrence after each cigarette at a characteristic interval. At first, a single cigarette can keep withdrawal at bay for weeks, but as addiction progresses, cigarettes must be smoked at progressively shorter intervals to suppress withdrawal symptoms. The physiologic need to repeatedly self-administer nicotine at shorter intervals explains a full spectrum of addictive symptoms ranging from the prodromal symptom of wanting, to chain smoking. The early process of nicotine addiction is recognized if a person experiences regular wanting for a cigarette. When symptoms include craving or needing, the now addicted patient is experiencing a compulsion to smoke. This simple diagnostic approach covers the full spectrum of addiction in smokers of all ages and levels of tobacco use, and is more valid than a clinical diagnosis based on the current Diagnostic and Statistical Manual criteria.