Membranes can be defined as a selective barrier between two phases and classified into biological or synthetic membranes. Synthetic membranes are subdivided into organic and inorganic membranes and made from a large number of materials including an organic polymer, ceramics, metal and glass. With regard to the presence of pores, membranes can also be distinguished into porous and non-porous membranes. The choice of membranes for practical applications is based on the membrane materials and membrane structures. Flow and selectivity are two basic key parameters for membrane process applications, which have successfully been applied over the last 40 years to different separation processes (usually, liquid mixtures). This book gathers the latest research from around the world in discussion of this dynamic field.