Release of toxic chemicals into the environment cannot always be avoided completely. As a result organisms, man included, will be exposed to chemicals via the environment. Given the release of certain chemicals into the environment, their exposure concentrations in air, water and soil would depend on the rates at which they are removed from the environment. This chapter deals with the transport and transformation processes that affect concentrations in the environment, with emphasis on the modeller's perspective. Being interested primarily in the effects that processes have on concentrations of chemicals in environmental media, we focus on a quantitative description of the rates at which losses from the environment take place, and on how these rates differ for different chemicals. We systematically formulate process rate constants for each transport or transformation process. Eventually, the rate constants combine into a mass balance model which allows us to describe and predict how releases into the environment result in exposure concentrations of organisms.