Statoil have been operating at Sleipner since 1996 marking twenty years of injection monitoring at the site. The monitoring programme has been shown to be a success proving both conformance and containment as well as including contingency plans. Repeated seismic and gravimetric surveys have been conducted throughout the lifetime of the project alongside acoustic imaging of the seabed. Statoil and partners have released all seismic data acquired up to and including 2008. Gravimetric surveying allowed initial estimates of the density of CO2 within the reservoir to be calculated and then later to determine an upper limit on the amount of CO2 dissolved in brine (when combined with other data). Key learnings highlighted from the past 20 years included the importance of controlling the injection in-situ conditions using downhole pressure and temperature gauges; repeated seismic surveys were crucial for ensuring containment monitoring; the combination of seismic and gravimetric data allowed an estimate of the amount of CO2 dissolution in water to be made and future dedicated monitoring plans should take into account higher frequency and resolution technologies now available.