Among solutions to contribute to CO2 emissions mitigation, sequestration is a promising path that presents the main advantage of being able to cope with the large volume at stake when considering the growing energy demand. Of particular importance, geological storage has widely been seen as an effective solution for large CO2 sources like power plants or refineries. Many R&D projects have been initiated, whereby research institutes, government agencies and end-users achieve an effective collaboration. So far, progress has been made towards reinjection of CO2, in understanding and then predicting the phenomenon and fluid dynamics inside the geological target, while monitoring the expansion of the CO2 bubble in the case of demonstration projects. A question arises however when talking about sequestration, namely the time scale to be taken into account. Time is indeed of the essence, and points out the need to understand leakage as well as trapping mechanisms. It is therefore of prime importance to be able to predict the fate of the injected fluids, in an accurate manner and over a relevant period of time. On the grounds of geology, four items are involved in geological storage reliability: the matrix itself, which is the recipient of the injected fluids; the seal, that is the mechanistic trap preventing the injected fluids to flow upward and escape; the lower part of the concerned structure, usually an aquifer, that can be a migration way for dissolved fluids; and the man-made injecting hole, the well, whose characteristics should be as good as the geological formation itself. These issues call for specific competencies such as reservoir engineering, geology and hydrodynamics, mineral chemistry, geomechanics, and well engineering. These competencies, even if put to use to a large extent in the oil industry, have never been connected with the reliability of geological storage as ultimate goal. This paper alms at providing an introduction to these interactions and examining the consequences of storing CO2 underground over long period of time. It is indeed of tremendous importance, if geological storage is to become an accepted solution from both a technical and social point of view, to focus from now on long term issues. (C) 2004 Published by Elsevier Ltd.