In 2008, the Practice Guideline “Troubleshooting of Wastewater Treatment Plants“ has been published for the first time by the Baden-Württemberg State Association of the DWA. The guideline became a useful tool for wastewater treatment plant operators, not only in Baden-Württemberg but in the entire German language area. For this reason and because of numerous inquiries from international experts this paper has been edited in english. The booklet can assist wastewater treatment plant operators in optimizing day to day operations and troubleshooting operational problems. Futhermore executives can improve their processes and efficiency with the aid of the guideline. The structure of the booklet is conceived to symptomatic features and therefore offers a fast access to the solution of a problem. The authors are accepted experts with essential practical experience in the design and operation of wastewater treatment plants.