Climate change already have widespread and significant impacts in Europe, which is expected to increase in the future. To reduce the damage, detailed assessments, based on a thorough understanding of the hydrological system, are required for the planning of optimal adaptation strategies. Groundwater plays a vital role for the inland freshwater cycle, and have the capability of buffering or enhancing the impact from extreme climate events causing droughts or floods, depending on the subsurface properties and the status of the system (dry/wet) prior to the climate event. Understanding and taken the hydrogeology into account is therefore essential in the assessment of climate change impacts. The Geological Survey Organisations in Europe acquire the necessary data and knowledge of the groundwater system and some Surveys already have high-end expertise in utilising this in climate change assessments. To streamline the assessments to produce harmonised results at EU scale, and to contribute to a general enhancement of the assessments, the Surveys will collaborate in TACTIC on the development of a research infrastructure for the advancement and harmonisation of climate change assessments utilising knowledge and data on the groundwater system, which is tested in pilots covering most climate challenges and hydrogeological conditions in Europe. Supplying data and results to a European Information Platform for storage and visualisation, TACTIC will further contribute to easy access of information relevant to climate change assessments, which may be used directly or integrated into future decision support systems.