The concept of fugacity, which is widely used in chemical processing calculations, has also been successfully applied to a variety of environmental simulations of the fate and transport of organic contaminants. The challenges of estimating fugacities in environmental phases are discussed, especially for ill-defined phases such as soils, sediments, and biota for which activity coefficients and molar volumes cannot be measured. It is shown that by lumping these quantities and a reference fugacity in a single parameter, empirical partition coefficient data can be used to deduce fugacities and thus the relative equilibrium status between phases and directions of diffusive transport. For assessments of substances that display narcosis, chemical activities, which can be readily deduced from fugacities, can provide valuable estimates of the proximity of calculated or measured environmental concentrations to potentially toxic levels. Five illustrations are presented to demonstrate the value of applying the fugacity concept in environmental contexts, namely, the equilibrium distribution of diverse substances, the evaluation of air-water exchange processes, bioconcentration and bioaccumulation in fish, comprehensive risk assessment of regional chemical fate and exposure, and demonstrating the global distribution of chemicals by atmospheric and oceanic transport.