The aim of monitoring injected CO2 is to demonstrate the effectiveness of a storage project and to check for any possible leakage. Migration of CO2 from the storage reservoir could possibly occur through poorly sealed and improperly abandoned wellbores or transmissive faults and fractures in the caprock. An escape of CO2 from storage could be detected through losses in the reservoir, migration in the rock above the reservoir and elevated CO2 concentrations in the surface environment. There is a range of monitoring techniques that can be deployed to monitor the migration of CO2 in the reservoir and detect leakage of CO2. For monitoring the reservoir seismic imaging and downhole pressure/temperature measurements are key tools;, gravimetry, electromagnetics and other techniques can also add useful information. For leakage monitoring onshore, surface or atmospheric techniques such as eddy covariance, open path lasers, soil gas flux and concentration measurements can be deployed. Leakage into the marine environment can be detected, and to a degree measured, using seabed and water-column acoustic imaging and sampling, water geochemistry, benthic chambers, and observation of seabed fauna. The success of any of these methods for accurately quantifying leakage from a CCS site will depend on their ability to locate and define the physical extent of a leak and their ability to accurately separate baseline (initial data collected prior to injection) from leakage flux rates. It is also worth noting that should a CO2 storage project leak it would be in line for financial penalties under various pieces of legislation.