Cigarette smoking during pregnancy is one of the most important modifiable causes of poor pregnancy outcomes in the US and doubles the risk of delivering a low-birth-weight neonate. Ten percent of pregnant women continue to smoke, with higher rates among women in lower socioeconomic groups and in those with depression. Behavioral treatments for smoking during pregnancy are modestly effective and the safety and efficacy of first-line medications for smoking cessation in the general population have generally not been evaluated for use during pregnancy. Bupropion may be efficacious to treat nicotine dependence in pregnant women because: 1) pregnancy increases nicotine metabolism, which reduces the efficacy of nicotine replacement therapy~ in contrast, bupropion is efficacious in treating nicotine dependence in fast metabolizers of nicotine~ 2) bupropion is currently used widely to treat depression in pregnant women which is a strong risk factor for continued smoking in this group~ and 3) pregnancy increases the activity of the CYP2B6 enzyme, potentially increasing bupropion metabolism and the conCentreation of hydroxybupropion, which has been associated with better smoking-cessation outcomes. Importantly, data indicate that bupropion can be used safely after the first trimester of pregnancy. The specific aims of the study are: 1) To conduct a 10-week, placebo-controlled trial of bupropion 300 mg/day in 360 pregnant smokers (13-24 weeks gestation) from low- income, under-served communities. We will monitor smoking behavior and adverse effects of the medication during treatment, with follow up for the remainder of the pregnancy (measuring length of gestation, rate of pregnancy complications, and birth outcomes), and at three months postpartum. We will use text messages to send pregnancy-relevant information to enhance medication adherence. 2) To assess changes in depression symptoms and craving as mediators of bupropion's effect on quit rates. 3): To explore genetic, metabolic, and social/behavioral moderators of the efficacy of bupropion on cigarette abstinence and safety. This will be the first adequately powered clinical trial of the efficacy and safety of bupropion for nicotine dependence in pregnant smokers. The trial is innovative in its use of text messaging to enhance treatment adherence and the prospective evaluation of mediators and moderators of bupropion therapy in pregnant women. Importantly, if bupropion is efficacious in pregnant smokers, its use could reduce the rate of poor pregnancy outcomes in the United States.