ULTimateCO2 will 1) significantly advance our knowledge of specific processes that could influence the long-term (LT) fate of geologically stored CO2 and 2) yield validated tools for predicting LT storage site performance. The 4-year collaborative programme will cover detailed lab, field and modelling studies of the main physical & chemical processes involved and their impacts in the LT: a) trapping mechanisms in the reservoir (structural, dissolution, residual, mineral), b) fluid-rock interactions and effects on mechanical integrity of fractured caprock and faulted systems and c) leakage due to mechanical & chemical damage in the well vicinity. Integration of the results will enable an assessment of overall LT behaviour of storage sites at regional scale in terms of efficiency & security, also including other important aspects, e.g. far-field brine displacement and fluid mixing.The LT prediction of CO2 evolution during geological storage will thus become more robust, not only by addressing the uncertainty associated with numerical modelling, but also by applying realistic contexts and scales. The latter will be ensured through close collaboration with at least two demonstration sites in deep saline sandstone formations: the onshore NER300 Ouest Lorraine candidate in France (ArcelorMittal GeoLorraine) and the offshore EEPR Hatfield site in UK (National Grid).ULTimateCO2 will develop recommendations for operators and regulators to enable a robust demonstration of the assessment of LT storage site performance. Scientific knowledge on the LT efficiency and safety of CO2 storage will be disseminated widely to a broad audience, so that not only operators of demo sites will benefit, but also other stakeholder groups, including policy makers and regulators, storage developers, investors, the scientific community, and representatives of the general public (NGOs and CCS initiatives), thus helping to improve public understanding.