Almost all physicians’ medical interventions are, in principle, “additive” therapies conducted by prescription of a drug, whereas apheresis is the opposite, a “subtractive” therapy. The principles of apheresis can be categorized into membrane fractionation, adsorption, and centrifugation. Diffusion, ultrafiltration, and microfiltration are key components of membrane fractionation. The main components of hemodialysis and peritoneal dialysis therapy are diffusion and ultrafiltration. However, microfiltration is the principal process used in apheresis because the goal of apheresis is to prevent the passage of red blood cells (RBCs), white blood cells (WBCs), and other formed elements through the first membrane and allow passage of select plasma proteins through the second membrane. Adsorption therapy involves two types of therapy: direct adsorption using whole blood and selective adsorption from plasma after separation by the first membrane. Centrifugation is used to separate cells for blood constituent donation, peripheral blood stem cell donation, and ulcerative colitis therapy. Ex vivo apheresis is applicable to cell-free and concentrated ascites reinfusion therapy (CART) for patients with ascites or pleural effusion from cancer, liver cirrhosis, or other illnesses. This chapter discusses filter selection for treatment of diseases by using microfiltration.