A minireview is presented concerning the use of cotinine as a tobacco-smoke exposure index. First, general considerations about methods for the determination of urinary cotinine are presented. Besides pure analytical aspects, this minireview considers major problems encountered in the establishment of threshold values that can be used to distinguish not only smokers from nonsmokers but also nonsmokers exposed or not exposed to environmental tobacco smoke (ETS). In addition, the use of urinary cotinine is illustrated in several situations where smoking status assessment is of interest. Such situations include evaluation of the impact of smoking cessation programs, monitoring of pregnancy and of other groups at risk, assessment of occupational exposure to industrial pollutants, validation of phase I clinical trials, and the control of life insurance candidates. The specific problem of ETS exposure assessment is briefly mentioned.