The LOCUS modelling package, which has been designed to assess the performance of an urban drainage system that is controlled in real time is presented. Besides the simulation of 'optimal' controlled systems, LOCUS offers the possibility to simulate local (or static) controlled systems as well (i.e. the present way of operation of most urban drainage systems). Since an identical system description is used in both cases, the difference between the results is only due to the way the system is operated and hence the effects of real time control can be quantified by comparing the results. The use of the model is illustrated by a simple example, which shows that it is worth investigating the potential of real time control before constructing extra storage in the system. For a small fictitious system with limited storage capacity at the downstream section it is shown that this potential is comparable to increasing the storage capacity by 1.5 mm at this particular section.