The primary purpose of this study is to evaluate the safety and efficacy of varenicline given for a twelve week treatment period in comparison with placebo and Zyban for smoking cessation.

Inclusion Criteria:

• Subjects must have smoked an average of at least 10 cigarettes per day during the past year and over the month prior to the screening visit, with no period of abstinence greater than 3 months in the past year.

Exclusion Criteria:

• Subjects who have used bupropion (Zyban or Wellbutrin) previously.