Experimental studies of the movement of molecules in solutions and molecular transport across artificial or biological barriers are used by the pharmaceutical scholar in a variety of contexts, ranging from simple diffusion and dissolution studies, to complex in vivo pharmacokinetic investigations. Movement of molecules in solutions and molecular transport across barriers may be described mathematically, and knowledge about these descriptions will aid in the design of experiments and interpretation of data. A number of textbooks explain the mathematical background necessary for transport studies (for example Schultz, 1980; Steen-Knudsen, 2002). In the present chapter, however, focus will be on presenting only the most commonly used equations and explaining the parameters involved, and the circumstances under which these equations can be applied. The aim of this section is thus to provide a basic framework of concepts describing transport of drug substances across biological barriers, hopefully enabling the reader to choose appropriate experimental models and data analysis for a given problem related to flux and permeability studies.