This article gives an overview on the legal proceedings brought against the authorities by the environmental NGO “Deutsche Umwelthilfe”. The NGO demands the updating or modification of air quality plans in regard to the measures that are supposed to ensure that the level of Nitrogen-Dioxide (NO2) in the ambient air of German cities does not exceed the limit value. In particular the article describes the court´s difficulties in effectively examining those measures or the predictions given on the future development of air quality as well as the specific significance of the principle of proportionality in connection with the possible ban of diesel-powered vehicles. Finally it explains the preconditions and advantages of court or out-of-court settlements. The article is dedicated to H. D. Jarass.