In hemodialysis therapy, the contact between blood and the dialysis membrane causes biological reactions such as complement activations, the activation of blood cells and the coagulation system. Since 1960s, research has been conducted to develop dialysis membranes with excellent blood compatibility that prevents such reactions from occurring. The first dialysis membrane developed was a regenerated cellulose (RC) membrane. However, since transient leukopenia was observed when RC membranes were used, cellulose acetate (CA) membranes, in which one, two and/or all three hydroxyl groups of the RC is/are masked by corresponding numbers of acetyl groups, are currently being used. Conventional CA membranes have a homogeneous structure; however, asymmetric cellulose triacetate membrane (CTA) with a dense inner surface of hollow fibers has also been developed recently, which made it possible to have smoother surface of the membrane. This may make it harder for proteins and platelets to adhere to the membrane. In addition to cellulosic membranes, synthetic polymeric membranes such as polyacrylonitrile (PAN), poly(methyl)methacrylate (PMMA), ethylene vinyl alcohol copolymer (EVAL), polysulfone (PSf), polyethersulfone (PES), and polyester polymer alloys (PEPA) have also been developed. Since AN69^®, one type of PAN membranes, has a strong electric negativity, it can adsorb and remove positively charged cytokines and complements. PMMA is well known to have adsorptive characteristic to β2-microglobulin and inflammatory cytokines. Recently, PMMA with weakened negative charge has been developed to inhibit platelet adhesion. PSf membranes have high solute and water permeabilities. In order to improve blood compatibility, PSf membranes coated with vitamin E or those with novel hydrophilic polymers have been developed. When selecting a membrane for treatment, much attention should be paid not only for solute and water permeabilities, but also for its blood compatibility.