In Germany, today about 24,000 combined sewer overflow tanks (CSO tanks) are in operation. For their dimensioning and design, German standards are available and respected. However, efficiency and performance of these costly structures are usually not known by the operator nor by the local water authority. Thus, a trend towards monitoring the overflow activity is observed. This paper points out the state-of-the-art in CSO overflow monitoring. Basic features of water level measurement as well as some plausibility checks for the data are shown. Evaluation of overflow data is rather difficult. Assessment of the overflow activity versus hydrological data, e.g. by comparison with simulation results, is costly. A much simpler way of evaluation is the ranking and rating of measured overflow activity. On the 8th ICUD 1999, already a paper on this subject was presented. Now, 6 years later, the database of the ranking has grown to more than 500 years of measurement.

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