There are thousands of constituents in cigarette smoke, which makes assessment of total systemic exposure to cigarette smoke challenging. Exposure to cigarette smoke constituents can occur through many other sources in addition to cigarette smoke. Measurement of a tobacco constituent or metabolite in a biological fluid (biomarker of exposure - BOE) as a surrogate of classes of smoke constituents is a practical approach for assessing exposure. Observations from a stratified, multi-center, cross-sectional study of 3,585 adult smokers and 1,077 non-smokers as well as open-label, parallel design clinical studies in adult smokers (n=411) will be shown. Exposure to tobacco specific constituents was measured from 24-hour urinary excretion of nicotine and five of its metabolites (nicotine equivalents, NE), total NNAL (metabolites of NNK), total N'-nitrosonornicotine (NNN) and plasma cotinine. In addition exposure to the non-tobacco specific constituents - benzene (S-phenyl mercapturic acid), aromatic amines by urinary 4-aminobiphenyl (ABP), 4-ABP hemoglobin adducts, o-toluidine, 2-aminonaphthalene, carbon monoxide (blood Carboxyhemoglobin), acrolein (3-hydroxypropyl mercapturic acid), 1,3-butadiene (MHBMA and DHBMA) and PAHs (total 1-hydroxypyrene and 3-hydroxy benzo(a)pyrene) was also measured. In addition, it was also observed that some of the biomarkers have poor discriminatory power (e.g. DHBMA, o-toluidine and urinary 4-ABP) and would not be suitable for estimating exposure in adult smokers. Based on this analysis it can be concluded that NNN and 3-OH BaP have limited utility as biomarkers for estimating exposure to TSNAs and polycyclic aromatic hydrocarbons respectively. It can also be concluded that based on the analysis of the relationships with the different biomarkers, NE may be considered as a surrogate biomarker of total cigarette smoke exposure.