Online hemodiafiltration (HDF) is an extracorporeal technique for solute removal in renal failure, which takes advantage of an enhancement of convective treatment by the large amount of ultrapure nonpyrogen dialysate being used for substitution of the ultrafiltered volume. It offers many advantages aside from its safe inflammatory profile, which is attributable to the use of ultrapure dialysate and highly biocompatible dialysis membranes. Due to an improved convective clearance, significantly increased removal of large or protein-bound uremic retention solutes can be achieved, with a potential benefit on cardiovascular morbidity and mortality. Recent observational data indicate that online HDF offers a survival advantage even after adjustment for comorbidity and dialysis efficiency. Research has been ongoing to maximize further the effectiveness of the technique by new technical innovations such as transmembrane-pressure feedback control or mid-dilution online HDF.