Blood microfiltration is used to separate whole blood into plasma and a more concentrated cellular fraction. Therapeutic plasmapheresis is used for the treatment of a variety of diseases and disorders characterized by the presence of abnormal proteins in the circulation that are believed to be involved in the progression of that particular condition. Membrane devices provide a particularly attractive alternative to centrifugal plasmapheresis. The chapter discusses the design of a rotating cylinder module. The most attractive module designs for blood microfiltration are the hollow fiber, open flat plate, and rotating (annular) systems. A. L. Zydney and C. K. Colton developed a model for red-cell lysis during blood microfiltration in which the red cells were assumed to rupture following their deformation into the porous structure of the membrane. In order to avoid red-cell damage and blood leakage, the rotation of the inner cylinder is accomplished using a magnetic coupling device, thereby eliminating the need for a rotating seal.