The capacity of soils to store organic carbon is finite and specific to each soil type and location. Its limit is roughly equal to the original content of organic matter in each soil when it was in the virgin state under natural vegetation. That content varies from less than 1% by mass in arid-zone soils to more than 50% in bogs and marshes. In many agricultural soils, the OM content of the topsoil ranges from 2 to 5%. The practice of agriculture generally depletes soil organic matter (owing to accelerated decomposition without replenishment and to erosion of the topsoil). Soil organic matter can be restored by revegetation, conservaton tillage, cover crops, judicious fertilization and water management, etc. Such practices must be promoted and maintained consistently for decades to achieve full potential. Quantitative assessment of the potential for carbon sequestration in soils requires critical examination of the scanty data and the contradictory, sometimes exaggerated, claims of advocates. In principle, the potential is real and likely to be significant. Policies to promote sequestration must, however, be based on developing and instituting sound methods of monitoring performance in the field.