We will recruit 35 non-smokers and 30 adult smokers who smoke > 15 cigarettes/day for more than two years and are 18-55 years old. We expect about half of them will be females. All participants will be screened for drugs of abuse and female participants will receive a pregnancy test. Each smoker will participate in four tDCS sessions, one for sham and three for real tDCS, each for one set of electrode montage. The first set is dorsal lateral prefrontal cortex (i.e., F3 and F4 of 10/20 EEG system) and neck. The second set is middle line prefrontal cortex (Fz), superior parietal cortex (Pz), and bilateral inferior temporal cortex (T3 and T4). The third set is left dorsal lateral prefrontal cortex (F3) and superior parietal cortex (P1), and right inferior temporal cortex (T4). The sequence of real and sham tDCS will be counterbalanced among participants. Each session will last for 20 minutes and there will be a minimum 48 hours between the two sessions. Participants will be required to be abstinent from cigarette smoking overnight for a minimum of 10 hours and arrive at our lab around 8:30 AM for each tDCS session. After arrival, each participant will provide a breath sample for detection of any alcohol in their system, an assay of CO and for verification of abstinence from cigarette smoking. We will place a nicotine patch (Nicoderm with 21 mg nicotine) on the back of participants. We will also ask each participant to chew a nicotine gum (Nicorette gum with 4 mg nicotine) for 30 minutes. The participants will perform a battery of computerized tasks including the N-Back working memory task, and an attention task involving watching a computer screen and pressing buttons to indicate more odd or even numbers displayed on the screen. In addition participants will complete the following questionnaires related to mood, nicotine withdrawal, mental state, possible side effects, and cigarette craving: Profile of Mood States (McNair et al 1971), Shiffman-Jarvik Withdrawal Scales (Shiffman & Jarvik 1976), and Urge To Smoke (Jarvik et al 2000), tDCS side effects questionnaire, Wisconsin Smoking Withdrawal Scale (Welsch 1999), Perceived Stress Scale (Cohen 1983), Spielberger State/Trait Anxiety Questionnaire (Spielberger 1970).