CO2 produced at the Sleipner gas field is being injected into the Utsira Sand, a major saline aquifer some 1000m beneath the North Sea. The injection plume is being monitored by geophysical methods. 3D seismic data were acquired in 1994, prior to injection, and again in 1999, 2001 and 2002; seabed gravimetric data were also acquired in 2002. The CO2 plume is imaged on the seismic data as a number of bright sub-horizontal reflections.. growing with time, underlain by a prominent velocity pushdown. Quantitative modelling is based on plume reflectivity largely comprising tuned responses from thin layers of CO2 trapped beneath thin intra-reservoir mudstones, with layer thicknesses being mapped according to an amplitude-thickness tuning relationship. Between the layers a lesser component of much lower-saturation, dispersed CO2 is required to match the observed velocity pushdown. However, reservoir temperatures are subject to significant uncertainty, and inverse models of CO2 distribution, based oil lower and higher temperature scenarios, call produce both the observed plume reflectivity and the velocity pushdown. Higher temperature models however require that the dispersed component Of CO2 has a somewhat patchy rather than uniform saturation. Analysis of the datasets suggests that accumulations of CO2 as small as 500 tonnes may be detectable under favourable conditions, providing a basis for setting leakage criteria. To date, there is in fact no evidence of migration from the primary storage reservoir.