Mass transfer is the net movement of mass from one location to another in response to applied driving forces. Mass transfer is used by different scientific disciplines for different processes and mechanisms. It is an important phenomena in the pharmaceutical sciences; drug synthesis, preformulation investigations, dosage form design and manufacture and finally ADME (absorption, distribution, metabolism and excretion) studies. In nature, transport occurs in fluids through the combination of advection and diffusion. Diffusion occurs as a result of random thermal motion and is mass transfer due to a spatial gradient in chemical potential or simply, concentration. However the driving force in convective mass transport is the spatial gradient in pressure (Fleisher, 2000). On the other hand, there are other variables influencing mass transfer like electrical potential and temperature which are important in pharmaceutical sciences. In a complex system mass transfer may be driven by multiple driving forces. Mass transfer exists everywhere in nature and also in human body. In fact in the body, mass transport occurs across different types of cell membranes under different physiological conditions. This chapter is aimed at reviewing transport across biological membranes, with an emphasis on intestinal absorption, its model analysis and permeability prediction.