Five different artificial organs or tissues are selected from our studies to show how membranes are used as biomaterials. They are artificial kidney, artificial lung, artificial skin, artificial pancreas, and artificial cornea. The membranes in these artificial organs are mostly employed to allow the selective permeation of some substances, similar to those in industrial applications. However, biocompatibility is additionally required in the medical applications of these membranes. It is shown that coupling of poly(ethylene glycol) onto the cellulose membrane used for hemodialysis greatly improves the blood compatibility, while graft polymerization of acrylamide onto a silicone membrane used for extracorporeal membrane oxygenation largely reduces clot formation on the surface. The silicone membrane is shown to be effective also for protection of damaged skin, when used with a porous collagen sheet. It is further demonstrated that poly(vinyl alcohol) membranes without and with immobilized collagen can be used for the entrapment of Langerhans islets and for the repair of damaged corneal tissue, respectively.