AbstractCO2 storage monitoring programmes aim to demonstrate the effectiveness of the project in controlling atmospheric CO2 levels, by providing confidence in predictions of the long-term fate of stored CO2 and identifying and measuring any potentially harmful leaks to the environment. In addition, the EU Emissions Trading Scheme (ETS) treats leakages of stored CO2 from the geosphere in to the ocean or atmosphere as emissions, and as such they need to be accounted for. An escape of CO2 from storage may be detected through losses from the reservoir, or migration through the overburden, into shallow groundwater systems, through topsoil and into the atmosphere, or through a seabed into the water column. Various monitoring techniques can be deployed to detect and in some cases quantify leakage in each of these compartments. This paper presents a portfolio of monitoring methods that are appropriate for CO2 leakage quantification, with a view to minimising both uncertainties and costs.