Health scientists and risk assessment experts are developing approaches to estimate exposure of human populations and ecosystems to environmental contaminants. Ecological scientists are exploring methodologies for estimating the exposure of ecosystems, or subdivisions within an ecosystem, to environmental stresses, while human health scientists are investigating approaches for estimating exposures to contaminants that can affect human health. Exposure assessment methods vary significantly, depending upon factors, such as the scale of the exposure, the measurement focus, and level of biological organization. The paper discusses the elements of ecological and human exposure assessment methodologies. Examples of multiple pathway exposure assessments are provided to illustrate human exposure concepts, and how they may also apply to ecosystem exposure assessments. Ecosystem and human exposure assessment paradigms are compared and contrasted with regard to the level of biological organization, source-receptor relationships, biomarkers, dose, pollutant characteristics, and modeling.