Evidence to date indicates that leakage is of low probability if site selection, characterisation and storage project design are undertaken correctly. In Europe, the Storage Directive (EC, 2009) provides a legislative framework, implemented by Member States, which requires appropriate project design to ensure the storage of CO2 is permanent and safe. However, it is incumbent on storage site operators to demonstrate an understanding of the potential impacts on surface ecosystems should a leak occur. The RISCS (Research into Impacts and Safety in CO2 Storage) project has produced a Guide to potential impacts of leakage from CO2 storage (the 'Guide'). RISCS assessed the potential effects of CO2 leakage from geological storage on both onshore and offshore near-surface ecosystems and on potable ground water. This assessment was achieved through laboratory and field experiments, through observations at sites of natural CO2 seepage and through numerical simulations. The Guide summarises some of the key findings of the project. The Guide provides information on the best approaches to evaluate potential impacts of hypothetical leakage from CO2 storage sites and to provide guidance on appraising these impacts. This information will be relevant to regulators and operators in particular, but also to other stakeholders who are concerned with CO2 storage, such as national and local governments, and members of the public.