This paper reviews the underlying assumptions behind environmental transport and food chain modeling of organic chemicals, and highlights the uncertainties affecting model applicability to exposure assessment. Limitations in: (i) our knowledge of fundamental transport and transfer mechanisms, (ii) parameter quantification, and (iii) model validation currently restrict most assessment applications of models to screening calculations. Simple multimedia partitioning and screening models identify major pathways of exposure or principal environmental sinks. Their use, however, is currently limited by a lack of accurate physicochemical data for the test chemicals. Exposure assessments for specific sources (e.g., landfill sites, incinerators, factory discharges) must evaluate the fluxes of emissions from the source to the receptor population via air, soil, surface and groundwaters, and the food chain. Current organic chemical transport models introduce many uncertainties to the exposure evaluation. This may be because the models are inherently unsatisfactory, inadequately describing the fundamental transport mechanisms likely operating in the field. Further limitations arise because some of the parameters are difficult to quantify accurately, because the environmental behavior of the test chemical is poorly understood, or because of temporal and spatial variabilities in field conditions. Until these processes are more fully understood, the usefulness of detailed simulation models for human exposure evaluations will be limited. Some media-specific models, notably of air and surface water transport, are mechanistically sound and considerably more sophisticated than the available input data. Emphasis should therefore be placed on field calibration and validation. Even a validated model, however, can not be routinely applied to other chemicals and/or at other locations.