The toxic effects of synthetic chemicals released into natural environments are a function of concentrations, of physico-chemical speciations, and of transformation products whose genesis is mediated by properties of the environment itself. Accurate evaluation of the probable consequences of particular releases requires an ability to forecast the speciation, transport, and transformations of chemicals. In aquatic systems, ionic and sorptive equilibria, advective and dispersive fluid transport, benthic uptake and release processes, volatilization, hydrolysis, direct and indirect photochemical processes, redox reactions, and microbial transformations have significant effects on the fate of introduced chemicals. Recent and continuing investigations of the kinetics and environmental determinants of these processes have made possible the design of models and computer codes that can generate theoretically sound forecasts of chemical events in ecosystems. These "fate codes," when coupled to equally rigorous techniques for computing effects of chemicals, can enhance the rationality, realism, and reliability of chemical safety evaluations.