Typical ‘bio’-molecules are produced in highly dilute concentrations in multicomponent aqueous systems in the presence of numerous co-solutes as shown in Figure 9.1. However, their applications usually demand the highest purity possible with levels of contamination typically in the ppm-range, especially for pharmaceutical products. The complexity of these target products in combination with that of the mixtures in which they occur, requires sophisticated separation processes. In particular for the efficient separation of components with a far-reaching structural similarity, such as stereoisomers and optical isomers, a substantial knowledge of the phase behavior is required. For many industrial processes, even those operated at a large scale, this knowledge is only available on an entirely empirical basis.