Four important factors must be considered in a systematic risk assessment of exposure to an environmental contaminant such as environmental tobacco smoke (ETS). These are hazard identification, dose-response determination, exposure assessment, and risk characterization. All of these are combined in a risk assessment. This presentation focuses on the exposure component. Only during the last few years have large-scale studies of peoples' exposure to ETS been conducted by new methodologies of personal monitoring. These techniques require few assumptions because they measure ETS exposure directly. Exposure is properly defined as the product of concentration times duration. Unfortunately, this simple concept is routinely abused. If either of these two factors is unknown, then exposure must be estimated based upon assumptions rather than direct measurement. Many attempts have been made to estimate ETS exposure with few or no analytical determinations. These include use of the dichotomous variable married or not married to a smoker, questionnaire responses, models, area concentration determinations, and finally, body fluid measurements. None of these is a direct measure and all require assumptions to derive an exposure value. In addition to the fact that few ETS exposure measurements exist, most of the controversial compounds in tobacco smoke are present in ultra-trace quantities that require heroic efforts to determine in real-life situations. Some researchers have attempted to increase the "scare value" of ETS measurements by relating concentrations of major, measurable components of ETS, e.g., nicotine to the unmeasurable controversial compounds through the use of assumed-constant sidestream smoke ratios. The fallacy of this assumption is discussed. This presentation reports on the techniques of personal monitoring, summarizes recent results, and provides a critique of improper methods that have been used to estimate nonsmoker exposure to ETS.