Water managers in the Rhine basin are increasingly confronted with information on the impact of climate change and climate variability on the discharge regime of their river system. The river Rhine has a long history of flooding events that caused casualties and severe damage. The question remains whether measures that are currently implemented upstream in Germany have either positive or negative effects on peak water levels downstream. Adaptation can significantly reduce impacts of climate change and is seen as an important part of societal response to global climate change. Planned adaptation implies decisions and measures within society that help to anticipate to climate related risks. The overall aim of ACER is to investigate the impact of climate change and to explore adaptation strategies for the Rhine basin under climate change, for both basin wide as well as regional water managers. The ACER project follows a so-called scenario analysis whereby solution trajectories are analyzed and compared, under the assumption of various long-term climate change and socio-economic scenarios. At the core of the ACER scenario approach are a group of stakeholders representing water managers at different levels in the Rhine basin. The results can be grouped in two themes: 1) Modeling processes - a trade-off. Instead of creating an overall new "Rhine model" for this project, we realized there is a trade-off between model complexity and the ability to run long time series and/or large ensembles of climate change scenarios. Considering model performance, focus and scale of the problem, a unique model set-up had to be optimized for each project objective. We coupled two complex models to simulate and investigate land surface - atmosphere feedback mechanisms. However, to simulate the combined effect of climate change and flood management measures, another model was developed that can be forced with output from a rainfall generator (10.000 year series of meteorological daily data). This model set-up aims to capture only the most relevant processes when assessing the effects on low-probability flood events, in a so-called "process-based" approach. 2) Effects of climate change and adaptation strategies. We project that climate change may increase flood probabilities with a factor 2.5 to 4.7 in 2050, which may strongly influence the expected annual losses due to flooding. The currently implemented and proposed measures in the Action Plan on Floods by the International Commission for the Protection of the Rhine (IKSR), as well as several additional measures we evaluated may be inadequate to cope with the increased flood probabilities. At the regional scale of a Dutch Water Board, the effects of climate change on polder hydrology is more intense than expected, caused by the dependence on both local climate conditions and water levels of the Rhine and Meuse rivers, which are mutually reinforcing.