[en] This publication presents the results of the RexHySS programme (research programme on the impact of climate change on water resources and hydrological extremes in the Seine and Somme river watersheds) which quantified the impact of climate change on the Seine water resource while taking the main uncertainty origins into account. Two scenarios of greenhouse gas emission, six climate models, three dis-aggregation methods and five hydrological models have been used. After a recall of the greenhouse effect principle, the report describes climate evolution and climate change (time history, climate modelling, assessment of climate models and debates about climate evolution, necessity of using dis-aggregation methods), proposes an overview of climate predictions (scenarios of greenhouse gas emission, climate prediction by the IPCC, dis-aggregated climate predictions used for the Seine watershed). It reports an assessment of the Seine basin climate and of its evolutions, and an analysis of the impact on the Seine watershed hydrology (hydrological models, impact studies, impact of climate change on water assessment, application to the Seine flow rates in Poses, impact of climate change on piezometry).