Mass can be transported in various ways, for example by truck, aircraft, or barge. Here the context is quite different; we shall use the term “mass transfer” to characterize the movement of molecules under the influence of a concentration gradient. Of special concern will be molecular movement across a phase boundary. The primary applications of such interphase transfer will involve gasliquid, vapor-liquid, liquid-liquid, and fluid-solid transfer for absorption/stripping, distillation, extraction, and adsorption. The principles described can easily be extended to other operations such as leaching and solids drying.