The way that carbon accounting is done greatly influences the value attributed to maintaining tropical forests. Accounting choices will be determining factors in the role that Brazil’s Amazon forest plays in global mitigation efforts and in the role that funds from mitigation will play in redirecting the course of history in Amazonia. Critical decisions include the form of accounting (stocks versus flows) that are applicable under different circumstances, baselines for establishing additionality, and the restrictions and adjustments (including discounting to attribute value to time) applied to reflect differences in permanence, leakage and uncertainty. None of these problems is insurmountable, but addressing them will require both academic effort and the political courage of decision-makers to act on available information.