This is a clinical study of the efficacy and safety of up to 52 weeks of varenicline therapy in conjunction with individual counseling for smoking cessation. Adult volunteers in generally good health, smoking 5 or more cigarettes per day, will receive 13 weeks of open-label varenicline therapy. At 12 weeks after their target quit date, they will be assigned in a random, double-blind manner to either 40 additional weeks of varenicline or placebo. It is hypothesized that biochemically-confirmed abstinence rates will be higher for the varenicline group at 52 weeks. Participants will be followed for an additional 26 weeks post-treatment.