Renal replacement therapy performed for end-stage renal disease patients now occurs almost exclusively with hollow-fiber dialyzers. Because renal replacement therapy utilizing such a device requires goals to be set with regard to the rate and extent of solute removal, a thorough understanding of the mechanisms by which solute removal occurs is necessary. This chapter provides an overview of solute removal by hollow-fiber dialyzers. In the fi rst part of the chapter, the major characteristics of hollow-fiber membranes influencing solute removal are discussed. Within this section, the chemical composition and physical characteristics of commonly used dialysis membranes and the features determining their solute permeability properties are reviewed. The remainder of the chapter emphasizes the major determinants of hollow-fiber dialyzer performance.