Chromatography consists of two distinct immiscible phases (the mobile phase and the stationary phase) and molecules will interact differently with each of these immiscible phases depending on their properties. The plate theory of chromatography assumes that the molecules in the mobile phase interact immediately with the stationary phase and that progress of molecules down the column is by sequential transfer from one theoretical plate to the next theoretical plate. The theories of chromatography are applicable to the chromatography of small molecules and proteins, but special considerations have to be made when the purification of proteins is attempted. Regardless of the equipment involved or the chromatographic technique that will be employed in the purification strategy, all chromatographic runs follow a set of separate but interrelated stages. Setting a target for the degree of purification required at the outset is applicable to both laboratory- and industrial-scale protein purification schemes.