This chapter briefly reviews mass transfer to complete the overview of the fundamental unit processes in pharmacy. Mass transfer is conceptually and mathematically analogous to heat transfer, as will be seen in the following exposition. Many processes are adopted so that a mixture of materials can be separated into component parts. In some, purely mechanical means are used. Solids may be separated from liquids by the arrest of the former in a bed permeable to the fluid. This process is known as filtration. In other examples, a difference in density of two phases permits separation. This is found in sedimentation and centrifugation. Many other processes, however, operate by a change in the composition of a phase due to the diffusion of one component in another. Such processes are known as diffusional or mass transfer processes. Distillation, dissolution, drying, and crystallization are examples of mass transfer processes. In all cases, diffusion is the result of a difference in the concentration of the diffusing substance. This component moves from a region of high concentration to a region of low concentration under the influence of the concentration gradient.