The nature of exposure assessment was reviewed and an approach was presented for integrating exposure assessment and epidemiology. An application of the joint approach was given. To adequately describe exposures in an occupational setting, the toxic agent or agents present must be identified, different degrees of exposure among workers and the reasons for these different exposure levels must be understood, and the changes in exposures over periods of time must be taken into consideration. The industrial hygienist must determine in terms of practicality what to measure, whom to measure, what locations to measure, and what time periods to include in the measurements. The use of both toxicokinetic and pharmacodynamic models was considered. A general approach was suggested which calls for identification of target tissues and route of entry of the offending agent, identification of possible agents and modes of exposure, development of a toxicokinetic model, development of a pharmacodynamic model, development of an exposure assessment strategy and collection of the data, and data analysis. As developed, this new approach provided a biological basis for exposure evaluation as part of an epidemiological study of an occupational health effect. The author states that this approach requires a much closer collaboration between the industrial hygienists and epidemiologists than has been true in the past and should be more successful in determining and evaluating subtle effects and dose response relationships for occupational exposures. One of the major outcomes of this approach will be its ability to provide for direct exploration of disease mechanisms.