In a given treatment modality, the performance characteristics of the dialyzer determine the quantity and nature of uremic toxins removed from the patient’s blood, provided that an adequate treatment time and flow conditions are prescribed. Dialyzer selection may be the most difficult task facing a dialysis facility. Practitioners must understand the functions of a dialyzer, membrane biocompatibility, implications of poor technique, financial and quality implications of dialyzer reprocessing, and matching the patient to the dialyzer’s capabilities. Dialyzer membranes are a vital contributor to the success or failure of hemodialysis therapies and hemodialysis adequacy. Matching a dialyzer to patient requirements is crucial to meet the prescribed clearance goals.