Objective of this synthesis report is to summarize the main achievements of the WELLMA-2 project. Based on the preparatory phase WELLMA-1 (2007-2009), the main project phase WELLMA-2 (2009-2012) included extensive laboratory, pilot-scale and field site investigations aiming at optimizing the operation and maintenance of drinking water production wells with respect to costs, energy efficiency and sustainability. The main reason for inefficient well performance is so-called well ageing. Deposit formation due to multiply correlated biological, chemical and/ or physical clogging processes in and around the well cause a decrease in performance. Thus, the interdisciplinary WELLMA-project team aimed at improving the efficiency of drinking water production wells by providing a scientific basis to support operators in their efforts to reduce well ageing. This included the development of guidance and recommendations for an adapted and well-planned operation scheme and maintenance strategy to sustain or reinstall the well performance. Well ageing processes were intensively studied at a multitude of vertical drinking water production wells located in Berlin, Germany and near Bordeaux, France. Thereby, classical monitoring and diagnosis methods, such as pumping tests and TV inspections, but also newly developed own experimental setups, such as the in-situ measurement of oxygen, depth-oriented water sampling or the exposure of object slides and bio-reactors for biofilm growth were applied. This synthesis report follows the project outline featuring four work packages dealing with (i) the identification of ageing types and the site-specific ageing potential from optimal data processing of site and well characteristics to provide decision support for the diagnosis and subsequent optimisation of well operation, monitoring and maintenance, (ii) field methods and experimental setups applied within the WELLMA-project to investigate mixing processes, oxygen uptake and biofilm formation, (iii) the impacts of intermittent operation on the uptake potential and distribution patterns of oxygen, and (iv) the efficiency of hydrogen peroxide treatments for preventive well maintenance against biochemically induced iron ochre formation and the oxygen uptake potential correlated to the decomposition of H2O2. Intermediate data were presented at various occasions at scientific and practiceoriented conferences, e.g. the Association for General and Applied Microbiology (VAAM), the International Water Association (IWA) Groundwater conference, International Association of Hydrogeologists (IAH), Berlin-Brandenburger Brunnentage, Wasser Berlin etc. and in related papers. A publication list is given at the end of this synthesis report.