On-line HDF represents a major technical development in the delivery of hemodialysis therapy: It combines the properties of increased diffusion available in current high-flux membranes with convective removal of between 6 and 30 liters per treatment and requires the use of ultrapure water and online filtration of replacement fluid. On-line HDF has been successfully introduced in Europe and Asia and is routinely prescribed for dialysis patients in these regions.The book at hand summarizes the history and achievements of on-line HDF in four parts: A report of the technological development in both machine and fiber/dialyzer is followed by a description of the challenges encountered in the evolution of on-line HDF, collecting the accounts of clinical key opinion leaders who had been involved in its early application. The third part presents a comprehensive review of the clinical results achieved with on-line HDF from its inception to the present times, in which it represents the clinical golden standard. The fourth and final part is dedicated to on-line HDF as a ‘vision’ for the future.