The amounts of CO2 that would need to be injected into geologic storage reservoirs to achieve a significant reduction of atmospheric emissions are very large. A 1000 MWe coal-fired power plant emits approximately 30,000 tonnes of CO2 per day, 10 Mt per year (Hitchon, 1996). When injected underground over a typical lifetime of 30 years of such a plant, the CO2 plume may occupy a large area of order 100 km2 or more, and fluid pressure increase in excess of 1 bar (corresponding to 10 m water head) may extend over an area of more than 2,500 km2 (Pruess, et al., 2003). The large areal extent expected for CO2 plumes makes it likely that caprock imperfections will be encountered, such as fault zones or fractures, which may allow some CO2 to escape from the primary storage reservoir. Under most subsurface conditions of temperature and pressure, CO2 is buoyant relative to groundwaters. If (sub-)vertical pathways are available, CO2 will tend to flow upward and, depending on geologic conditions, may eventually reach potable groundwater aquifers or even the land surface. Leakage of CO2 could also occur along wellbores, including pre-existing and improperly abandoned wells, or wells drilled in connection with the CO2 storage operations. The pressure increases accompanying CO2 injection will give rise to changes in effective stress that could cause movement along faults, increasing permeability and potential for leakage.Escape of CO2 from a primary geologic storage reservoir and potential hazards associated with its discharge at the land surface raise a number of concerns, including (1) acidification of groundwater resources, (2) asphyxiation hazard when leaking CO2 is discharged at the land surface, (3) increase in atmospheric concentrations of CO2, and (4) damage from a high-energy, eruptive discharge (if such discharge is physically possible). In order to gain public acceptance for geologic storage as a viable technology for reducing atmospheric emissions of CO2, it is necessary to address these issues and demonstrate that CO2 can be injected and stored safely in geologic formations. This requires an understanding of the risks and hazards associated with geologic storage, and a demonstration that the risks are acceptably small or can be mitigated. Much work is currently underway to develop comprehensive approaches towards risk assessment from a systems analysis perspective, which in general requires a simplified description of physical and chemical processes (Maul, et al., 2004, Espie, 2004; Wildenborg, et al., 2004; Walton, et al., 2004). This type of approach is very important, but needs to be complemented with development of an understanding of the physical and chemical processes associated with CO2 storage and leakage (Evans, et al., 2004).