Among people diagnosed with HIV/AIDS, the widespread use of highly active antiretroviral therapy (HAART) has greatly improved survival rates and changed the leading causes of death, from AIDS-related diseases to cardiovascular disease and lung cancer. Rates of tobacco use among individuals with HIV/AIDS are very high and varenicline may be particularly efficacious for treating nicotine dependence among individuals with HIV/AIDS. Through this trial, 310 smokers with HIV/AIDS will be randomized to varenicline plus 9 weeks of smoking cessation counseling or placebo plus 9 weeks of smoking cessation counseling. The investigators hypothesize that 1) varenicline and counseling will significantly increase end-of-treatment (week 12) and 24-week biochemically-confirmed abstinence, versus placebo and counseling; 2) quality of life will be rated higher in the varenicline and counseling group versus the placebo and counseling group, and there will be no significant differences between treatment arms in terms of the frequency of severe varenicline-related side effects; and 3) improved affect and reduced cognitive impairment will mediate the effect of varenicline therapy on quit rates.