The purpose of this chapter is to review terrestrial biological carbon sequestration and evaluate the potential carbon storage capacity if present and new techniques are more aggressively utilized. Photosynthetic CO2 capture from the atmosphere and storage of the C in aboveground and belowground biomass and in soil organic and inorganic forms can be exploited for safe and affordable greenhouse gas (GHG) mitigation [Watson et al., 2000]. Nevertheless, C sequestration in the terrestrial biosphere has not been seriously pursued since its introduction in the Kyoto Protocol over a decade ago. Concerns have been raised that C sequestration in the biosphere is finite and not permanent, that it is difficult to measure and monitor, that there would be carbon leakage outside of the mitigation activity, and that any attention paid to environmental sequestration would be a distraction from the central issue of reducing GHG emissions from energy production and use. International accord and success in reducing emissions from the energy system are not coming easily, and concerns about climate change are growing. It is time to reevaluate all available options that might not be permanent yet have the potential to buy time, bridging to a future when new energy system technologies and a transformed energy infrastructure can fully address the climate challenge. Terrestrial sequestration is one option large enough to make a contribution in the coming decades using proven land management methods and with the possibility that new technologies could significantly enhance the opportunity.