It is often not clear what the best measures of exposure are for a risk assessment, or even how one should answer this question. Environmental tobacco smoke (ETS) provides a good example for an exploration of uncertainty. There are a variety of methods for estimating exposure and each has short-comings. In this paper we summarize the physical characteristics of ETS and the principal methods for assessing exposure. We review the accuracy and applicability of these methods, and explore major sources of uncertainty in the assessment of ETS.