Outcome measures for smoking cessation are reviewed and evaluated, including 3 self-report measures and 3 biochemical validation measures. Point prevalence reflects the percentage of participants taking action, prolonged abstinence reflects those in the maintenance stage, and continuous abstinence reflects those who progress from action to maintenance without lapsing or relapsing. Biochemical assessments are primarily measures of point prevalence abstinence. The desirability of biochemical validation is a particularly controversial and critical issue. Three factors affect the accuracy of self-report: Type of Population, Type of Intervention, and Demand Characteristics. False-negative rates are generally low. Three broad issues impact on decisions to use biochemical validation: (a) alternative explanations for false positives, (b) refusal rate problems, and (c) the effect of inaccuracy on intervention assessment.