If deep reductions in anthropogenic greenhouse gas emissions are to be achieved, the introduction of CO2 capture and storage in geological reservoirs is likely to be necessary. The technology would be deployed alongside other mitigation measures such as renewables, energy efficiency and fuel switching. Currently, research programmes on the geological storage of CO2 are underway in the United States, the European Union, Australia and Japan. The aim of this paper is to present an overview of the research work that is currently underway and provide an analysis of the current state of knowledge on geological storage of CO2. The analysis will be broken down to address the key geological storage options: deep coal seams, depleted hydrocarbon reservoirs and deep saline aquifers. In each case, areas of uncertainty will be highlighted as well as areas where it is considered that further work will be needed so that the technology can be accepted by Governments and the general public as a mitigation option suitable for wide-scale application throughout the world.