A new global climate agreement will be most effective if parties are confident that it enables them to assess how well others are fulfilling their obligations. This can be achieved through a rigorous system of measurement, reporting, and verification. Key elements should include: annual emission inventories for all major greenhouse gas-emitting countries; national verification of mitigation commitments and actions in accordance with international guidelines; regular reports from parties detailing their implementation and verification of their commitments and actions; and expert review of parties’ inventories and implementation reports. Beyond verification, a new agreement should provide for a clear determination of whether a party is in compliance with its commitments. The compliance approach should be largely facilitative, rather than punitive, geared toward helping to identify and overcome obstacles to implementation.