Milk is a constituent of many food products. The functionality of the various components in milk could be used more effectively if they were available separately. Fractionation of milk components enables a more constant quality of products (for example, cheese) and the development of new products such as bioactive peptides. Therefore, milk fractionation will lead to a more efficient and diverse use of milk. During the past 25 years, dairy products and dairy processing have seen many changes. The most important change that has probably influenced the dairy industry is the development of membrane technology. Membrane separation technology seems to be a logical choice for the fractionation of milk, because many milk components can be separated according to their sizes and/or charges.