Internal hemodiafiltration is a high-flux bicarbonate dialysis modality with a special filter having geometric characteristics to increase the internal filtration. Internal hemodiafiltration improves convective transport by direct filtration and back-filtration without the use of a reinfusion fluid as in on-line hemodiafiltration. The back-filtration or passive reinfusion is predictable and can be calculated with a user-friendly software program. Internal hemodiafiltration not only has good capacity for intradialytic removal of low and middle molecules (similar to on-line hemodiafiltration), but can also reduce or even stop the steady increase in uremic toxins seen in standard bicarbonate dialysis. This paper describes the most important technical characteristics and clinical results of internal hemodiafiltration.