AbstractAt Ketzin, located west of Berlin, the GFZ German Research Centre for Geosciences operates Europe's longest- running on-shore CO2 storage site. The Ketzin pilot site has been developed since 2004 and comprises three wells to depths of 750 m to 800 m and one shallow observation well, an injection facility and permanently installed monitoring devices. Since June 2008, CO2 is injected into 630 m to 650 m deep sandstone units (Upper Triassic Stuttgart Formation) in an anticlinal structure of the Northeast German Basin. Until mid of May 2012, about 61,400 t of CO2 have been stored safely. One of the most comprehensive monitoring concepts worldwide is applied and capable of detecting the behaviour of the CO2 in the subsurface. The Ketzin project demonstrates safe CO2 storage in a saline aquifer on a research scale and effective monitoring. This paper summarizes the key results obtained after four years of CO2 injection.