When a membrane filtration process such as ultrafiltration is used a flux- and yield-decline can be observed. The causes are i) concentration polarization (i.e. accumulation of retained solutes, reversibly and immediately occurring) and ii) fouling phenomena such as adsorption, pore-blocking and deposition of solidified solutes, a long-term, and more or less irreversible process. The result of both these phenomena are a decreasing driving force for the filtration or an increasing resistance against transport of the permeating solvent during the filtration. The degree of flux decline depends on many variables, both solution and equipment related. Several models have been developed to describe the polarization phenomena, in general they can be subdivided in (A) resistance models, (B) gel-polarization models and (C) osmotic pressure models. A new boundary layer resistance model for unstirred dead-end ultrafiltration is described more in detail. This model can predict fluxes and related phenomena; the simulations agree very well with the experimental data. The flux decline behaviour of binary mixtures of equally and unequally charged proteins (α-lactalbumin, BSA and lysozyme) was studied. In case the mixture consists of oppositely charged proteins a considerable increase of the resistance of the concentrated layer near the membrane interface can be observed, which depends on the mixing ratio of the proteins. When equally charged proteins are filtered the resistance decreases a little, again depending on the mixing ratio. Several methods exist to improve the flux, they can be generally divided into: (1) adapting the operation conditions in the existing equipment, (2) altering the conditions in the solution, (3) using a different or pretreated membrane, (4) taking additional measures to prevent or decrease the flux decline.