In recent years, methods of on-line urea concentration and on-line urea clearance monitoring have been proposed for control of dialysis dose (Kt/V) and protein catabolic rate (PCR) in patients on regular dialysis therapy; these offer an alternative to the established methods of urea kinetics based on pre- and post-dialysis measurements of urea concentration. In contrast to such conventional urea kinetics, the new methods deliver results in real time and treatment parameters can be changed instantly. Three on-line measurement methods are to be distinguished: monitoring of urea concentration in ultrafiltrate, monitoring of urea concentration in dialysate (both yield Kt/V and PCR), and monitoring of urea clearance based on conductivity measurements. Some of these approaches are already applied commercially. Here, these methods are compared using results obtained from laboratory and clinical studies. The on-line methods are found to be more accurate than methods based on pre- and post-dialysis urea concentrations, and to be better suited for clinical routine. This paper outlines the principal methods, reviews the present literature, gives an overview of the applications and compares them to conventional pre- and post-dialysis concentration-based methods of urea kinetics. It is concluded that these methods are likely to find a widespread application in the control of dialysis adequacy.