To function as an effective method to avoid build-up of anthropogenic carbon dioxide (CO2) in the atmosphere, storage of CO2 in geologic repositories must be designed and operated to minimize leakage. The design objectives include the obvious requirement to avoid large sudden releases and the more challenging task of keeping long-term leak rates to de minimis levels. Long-term retention requirements are a function of both the atmospheric stabilization target for greenhouse gases and the degree of reliance on CO2 capture and storage as a mitigation measure. A robust strategy will employ the CO2 capture an storage method as a component of a portfolio of programs that emphasize energy efficiency and use of renewable energy. To preserve options to stabilize greenhouse gases at prudent levels, it appears we will need to rely extensively on all three components of this portfolio. In developing criteria for management of leakage from geologic repositories, the policy and technical community will need to engage with environmental groups and the public at the earliest possible point in the process. (C) 2004 Elsevier Ltd. All rights reserved.