Continuously operated chromatographic processes like the Simulated Moving Beds (SMB) are well established for the purification of hydrocarbons, fine chemicals as well as pharmaceuticals. With respect to discontinuous batch chromatography they have proven their ability to improve the process performance in terms of productivity, eluent consumption and product concentration especially for larger production rates. These processes are operated under isocratic conditions where the composition of the mobile phase remains constant. Non isocratic processes are necessary if e.g. bio-products like proteins with a large difference in their affinity have to be separated. Therefore the implementation of solvent gradients is an important approach to improve the performance of SMB processes. The advantages of solvent gradient SMB processes are achieved by a higher complexity with respect to layout and operation which makes an empirical design nearly impossible. Therefore process simulation based on validated rigorous models is necessary for process design and optimisation.