The assessment of CO2 storage sites is similar in many ways to reservoir characterisation in the oil industry: an integrated team of geoscientists and engineers is required to collect and analyse data, generate models and perform flow simulations in order to make predictions. The main difference, in the case of storage in saline aquifers, is that there is usually less geological and petrophysical data available. It is therefore useful to know if storage assessments will be adversely affected by this lack of data. The CASSEM project (CO2 Aquifer Storage Site Evaluation and Monitoring), was initiated in 2008 to address this issue by studying two analogue storage sites in the UK. Although CO2 storage may not be undertaken at these sites, similar formations off-shore will likely be used for CO2 storage in the future. The two sites were modelled at three levels, proceeding from simple models based on little data to more complex models using more detailed geological data and simulating geomechanical and geochemical processes. The results of each level of modelling were compared in order to measure the effect of increasing model complexity. The conclusions from this work are that, in order to model CO2 storage accurately, a significant amount of geological information is required and that an integrated approach to reservoir characterisation for CO2 storage is very important. It is also very important to consider the geomechanical effects, both during the injection period and for several decades after injection has ceased.