Background The Water Framework Directive (WFD) requires EU Member States to define programmes of measures (POM), or river basin management plans, that outline the main strategies for improving the ecological status of surface waters. Consequently, there is a need for scientific accompanying actions that will guide and benchmark proposed measures, because very few tools previously existed that could evaluate proposed measures in terms of their impact on the ecological status of surface water. Objectives The M3 project aimed to test in the field a combination of monitoring and modelling approaches that could inform decision-making processes concerning cost-effective measures for managing river basins in accordance with the WFD. Scientific concepts were to be tested to optimise their potential for supporting decision-makers during the planning and monitoring of the efficiency of river basin management measures in Luxembourg as well as the Dutch Delfland region and German Erft river basin. Specific objectives included demonstrating monitoring techniques that went beyond grab sampling techniques in terms of high-frequency sampling, continuous monitoring (passive sampling), sampling and analysis of different phases (solution, solids), flow component analysis and monitoring design using these techniques in an optimal fashion for the quantification of different pressures (e.g. included nutrients, pesticides, urban emissions like metals, PAHs and pharmaceuticals). Project funds were also used to test the applicability of emission models in catchments ranging from yearly substance flow analysis tools to process related dynamic models running on a daily or even sub-daily basis. Other proposed project features involved: carrying out scenario-building and outcome forecasts for river basin management measures; and demonstrating the usefulness of Water Quality models with a classical transport-chemical fate model for pollutant propagation and concentration prediction, as well as an ecological-ecotoxicological model that would be able to predict effects on food webs in surface waters. Results Most of the monitoring techniques introduced by the project fulfilled their purposes and delivered the expected results. Interesting opportunities were explored especially in the passive sampling domain and automated sampling devices. The combination of different methods in case studies was particularly fruitful. Challenges occurred during calibration and application of modeling tools in some case studies. For instance, the transferability of models like DatenFluss and DWA-WQ (which had been calibrated in the Erft region over years with substantial efforts) to Luxembourg was hampered because of missing datasets that could not be gathered in the short project life. Similarly, the application of the process-based models from scratch showed weaknesses in datasets that were not initially expected (mainly hydrological calibration data). Once the necessary confidence in the models has been established through multiple verifications and refinements, the sound testing of scenarios is possible. Both the beneficiary (CRP Tudor) and the partner (Erftverband) have generated an impressive number of follow-up projects that build on the results of the different M3 actions. These follow-up projects focus especially, but not exclusively, on the application of the Aquatox model and the modeling of emerging pollutants. Further information on the project can be found in the project's layman report (see "Read more" section).