The paper presents the development of a real-time expert system (RTES) for supervision and fault detection in urban drainage systems (UDS). Several vital characteristics of such an expert tool, in which the project is being developed, are recalled. The main aspects of the modelling of the UDS and the detection of faults are outlined and illustrated for the case of a test urban area, north-east of Paris. Procedural knowledge about the behaviour of the UDS is implemented using formulae and procedures. Object-oriented representation enables structuration and formulation of the UDS structure. Rule bases containing expert, heuristic knowledge are written in a quasi-natural language. Knowledge formulation and realtime implementation are illustrated on a RTES-prototype designed with G2 (Gensym Corp.).