Biochar is the charred by-product of biomass pyrolysis, the heating of plant-derived material in the absence of oxygen in order to capture combustible gases. The objective of this report was to review and evaluate published studies with regard to what evidence and arguments currently exist that assess the application of biochar to soil to a) sequester carbon and b) produce secondary agronomic benefits. Current analyses suggest that there is global potential for annual sequestration of atmospheric CO2 at the billion-tonne scale (109 t yr-1) within 30 years. So far, however, the underlying published evidence arises mainly from small-scale studies that do not currently support generalisation to all locations and all types of biochar.