The increasing consumption of drinking water puts a heavy burden on our future water resources. Therefore the reuse of rain water in households can be a good option to tackle this problem. On the other hand the rapidly drained rain water leads to problems in the combined sewer systems and watercourses. If the storage in the rain water tanks can be used to flatten the rain water runoff, rain water tanks can have an additional benefit. The effect of rain water tanks on the combined sewer overflow (CSO) emissions is therefore investigated with a reservoir model. Compared with storage in the combined sewer system or at the overflow, storage in rain water tanks will be more efficient in reducing the overflow emissions. However much more storage in rain water tanks must be provided to obtain the same overflow frequency as when downstream storage is used, because the storage in rain water tanks is less frequently available. When all the economical, social and environmental aspects are considered, rain water tanks can certainly be promoted as a good solution.