This protocol is intended to provide information regarding the efficacy and safety of the nicotine partial agonist varenicline tartrate, at a dose of 1 mg twice daily, for smoking cessation in diabetic subjects who smoke. Given that a better understanding of predictors of smoking cessation can be useful in identifying potential quitters and likely relapsers and that little is known about these predictors in diabetics, the role of different predictors of abstinence at the end of the study will also be examined Study Population The study will enroll 150 type 2 diabetic patients (≤ 75 years) who are regular smokers (≥10 cigs/day) and motivated to stop smoking in each of 2 treatment arms (active drug and placebo) Study Design The study is a double-blind, placebo-controlled, randomized clinical trial designed to assess the efficacy and safety of varenicline 1 mg BID in comparison to placebo for smoking cessation. The duration of active treatment will be 12 weeks and subjects will be followed in the nontreatment phase for an additional 12 weeks. This clinical study has an optional research component to prolong the follow up in the nontreatment phase for a full year. Predictors of abstinence at the end of the study will also be examined Study Endpoints Primary Endpoint: Success rates at week 24 in the varenicline vs placebo group. Success rates will be defined as the Continuous Quit Rate since last visit. Subjects will be classified as responders if they are able to maintain abstinence from cigarette smoking during this period of time with end-expiratory exhaled CO measurements ≤ 10 ppm. This measure will be obtained through reports of cigarette use by means of the Nicotine Use Inventory confirmed by a measurement of an end-expiratory exhaled carbon monoxide concentration that is ≤ 10 ppm on the study visit at week 24 Co-primary endpoint: Success rates at week 12 in the varenicline vs placebo group. Success rates will be defined as Continuous Quit Rate for Weeks 8 to 12 of treatment. Subjects will be classified as responders if they are able to maintain complete abstinence from cigarette smoking in each of the last four study visits (week 9, week 10, week 11, and week 12) with end-expiratory exhaled CO measurements ≤ 10 ppm. This measure will be obtained through reports of cigarette use by means of the Nicotine Use Inventory during the last four study visits (week 9, week 10, week 11, and week 12) confirmed by a measurement of an end-expiratory exhaled carbon monoxide concentration that is ≤ 10 ppm on each study visit Secondary Endpoint: Success rates at week 52 in the varenicline vs placebo group. Success rates will be defined as the Continuous Quit Rate throughout the last three visits (week 24, week 36, and week 44). Subjects will be classified as responders if they are able to maintain abstinence from cigarette smoking during this period of time with end-expiratory exhaled CO measurements ≤ 10 ppm. This measure will be obtained through reports of cigarette use by means of the Nicotine Use Inventory during the last three study visits (week 24, week 36 and week 44) confirmed by a measurement of an end-expiratory exhaled carbon monoxide concentration that is ≤ 10 ppm on each study visit Additional Measures: Given that a better understanding of predictors of smoking cessation can be useful in identifying potential quitters and likely relapsers and that little is known about these predictors in diabetics, the role of different predictors of abstinence at week 24 and at week 52 will also be examined