During the last 20 years, control of wastewater treatment plants has developed from very simple to advanced computer control methods based on on-line measurements. Fortunately, both on-line equipment and computer system technology are still developing fast and have become applicable at WWTPs. The use of on-line measurements for real time control is advantageous at most plants over 15.000 personal equivalent performing biological nutrient removal. Control strategies have demonstrated their efficiency at several Scandinavian wastewater treatment plants. Experience shows that the applied control strategies are efficient and robust for practical implementation, provide savings in energy and chemical consumption and decreases the nitrogen content in the effluent. Experience from a recirculating BNR plant using the STAR concept for advanced real time control is described. The on-line nutrient measurements and excitation of the processes through control strategies have improved the process understanding. Control has reduced the consumption of energy by about 30% and eliminated the need for external carbon addition, as well as improved the effluent quality of total nitrogen from the plant. The new measurements and data handling methods will make it possible to perform dynamic identification of activity in recirculating plants (Nielsen et.al., 1994), and hence give information as to which control strategies improve the biomass activity or favour desired cultures of micro-organisms.