Quantitation of exposure to tobacco products is useful for any individual smoker, and necessary for epidemiological studies which relate smoking to pathology, or which are concerned with the efficacy of smoking cessation methods. The medical history, trying to quantitate the current mean daily cigarette consumption (consumption rate), the cumulative risk (pack years) and the various types of smoking, including inhalation habits, should also be attempted. However, due to the quasi-systematic underrating of tobacco consumption which smokers have revealed on many occasions, together with difficulty in correctly observing cigarette smoking, objective validation of recent historical data by quantitative measurement of tobacco products in tissue fluids is mandatory. Measurements of nicotine and cotinine levels in serum and urine require elaborate and expensive methods, and are not adequate for validation of smoking cessation in smokers who are using nicotine chewing gum. Carbon monoxide is a good marker of smoke inhalation. Normal carboxyhaemoglobin levels allow us to confirm that a subject has recently stopped smoking, as its half-life is only a few hours in blood. Salivary (or plasma) measurements of thiocyanate discriminate between smokers and ex-smokers, with a high probability, particularly in those who have stopped smoking for at least 14 days, separating these clearly from current smokers. However, several potential causes of error must be considered to allow correct interpretation of measurements of thiocyanate.