This chapter focuses on some of the practical considerations of exposure assessment, with particular emphasis on atmospheric dispersion. The magnitude of exposure is determined by measuring or estimating the amount of an agent available at the body’s exchange boundaries during a specified time period. The exposure estimates generated by the model are used as inputs to physiologically based pharmacokinetic models that predict how a chemical moves through and concentrates in human tissues and fluids, and how long it will take for the chemical to be naturally processed and excreted from the body. Enormous amounts of waste discharged into water systems are degrading water quality and causing increased human health problems. The best physical model that can be used to describe a stream or river is a tubular flow reactor. The governing equations become 1-D in the Cartesian coordinate system in the direction of the moving water.