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This paper reviews quantitative assessments of uncertainty in level and trend in national greenhouse gas inventories. The reported uncertainty in the total emissions of high-quality greenhouse gas inventories ranges from ±5–20% in studies of five industrialised countries. The differences in uncertainty are, in particular, due to different subjective assessment of the uncertainty in emissions of nitrous oxide from agricultural soils. The fraction of CO
2 in the inventory has little effect on the uncertainty. The uncertainties in trends are about ±4–5 percentage points for those countries that have made estimates. High uncertainties of emission levels indicate potential for improvements and, consequently, recalculations. Recalculations will reduce uncertainty, but might also cause practical problems. A high uncertainty in the emission level for large emission sources may be an obstacle for assessing cost-effective reduction strategies as well as for designing effective systems of emission trading. This could imply that the more uncertain emission sources should be excluded from emission trading. Alternatively, subjective uncertainty estimates may be expressed in terms of an economic risk of recalculation. The latter system may allow a market-based encouragement to reduce emission uncertainty. Reductions in uncertainties are anticipated in the future. However, it will be extremely difficult to reduce the trend uncertainty. Trend uncertainties may consequently remain high compared with the emission reduction targets in the Kyoto protocol.