Therapeutic apheresis is a relatively new modality. Its absolute indications are few and include hyperviscosity syndrome, cryoglobulinemia, thrombotic thrombocytopenic purpura, Goodpasture's syndrome, and life-threatening complications of immunologic disorders refractory to conventional management. The use of apheresis in most of the other disorders discussed in this monograph is experimental and should not be employed unless all the mitigating therapeutic considerations clearly suggest an overwhelming advantage of apheresis. The promise of apheresis is much greater than its current use, and the research applications of specific component separation and antibody removal are of great importance. It is hoped that these new developments will shortly make current devices obsolete and improve the clinical management of patients as well as increase our knowledge of disease etiopathogenesis.