Haemodialysis is an extracorporeal process in which the blood is cleansed via removal of uraemic retention products by a semipermeable membrane. Traditionally, dialysis membranes have been broadly classified on the basis of their composition (cellulosic or noncellulosic) and water permeability (low flux or high flux). However, advances in materials technology and polymer chemistry have led to the development of membranes with specific characteristics and refined properties that mandate a reconsideration of traditional membrane classification systems. For adequate characterization of these newer types of membranes, additional parameters are now relevant, including new permeability indices, the hydrophilic or hydrophobic nature of membranes, adsorption capacity and electrical potential. In this Review, we provide clinicians with an updated analysis of dialysis membranes and dialysers. We discuss the basic mechanisms that underlie solute and water removal in dialysis (that is, diffusion, convection, adsorption and ultrafiltration) in the context of treatments that use highly permeable membranes. Specifically, we highlight online haemodiafiltration and new therapies (for example, expanded haemodialysis) that utilize membranes designed to produce a high degree of internal filtration. Finally, we discuss the considerations that govern the clinically acceptable balance between large-solute clearance and albumin loss for extracorporeal therapies.