Membrane based medical therapies have increased considerably in recent years. At the same time, awareness of the side-effects caused by blood material incompatibility has grown. The necessity of mass production at low cost has also contributed to the formation of demands placed on any membrane used in medical applications today. These demands are application specific transport characteristics, high clearance, blood compatibility and a design enabling cost efficient ways of production. A theoretical approach relating membrane structure to transport properties suggests that the deciding factors are pore radius, porosity, tortuosity, diffusion coefficients, pore shape and protein adsorption potential. A hollow fiber structure, which fits these demands for hemodialysis, hemodialfiltration and hemofiltration, is a three-layer structure consisting of an inner blood facing skin layer, followed by a sponge structure and a macroporous finger structure. The influence of the membrane surface on protein adsorption, leading to a change in permeability, has also been investigated. Two concepts for limiting protein and cell interaction with artificial surfaces are introduced. Copolymerization and blending of hydrophobic and hydrophilic polymers have been successful approaches. To limit blood-membrane interactions, an optimized microdomain surface structure formed by hydrophilic patches in a hydrophobic matrix has evolved. In vitro test methods for measuring activation levels of coagulation, kallikrein-kinin pathways, cell stimulation and complement activation were necessary for the development of highly optimized artificial membranes made for hemodialysis, hemodiafiltration and hemofiltration. The Polyflux S membranes, consisting of the hydrophobic polymers polyamide and polyarylethersulfone as well as hydrophilic poylvinylpyrrolidone, with their integral three-layer microdomain structures performed well in all of the mentioned in vitro tests.