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This paper provides a summary of our current understanding of the key drivers of N2O emissions from soil in temperate and tropical, natural and agricultural ecosystems. These drivers are substrate supply, as N additions and mineralisation of organic N in soil, soil water content and temperature. They can exert synergistic or antagonistic influences on the emissions which can vary spatially and temporally. Such influences explain why emission rates often differ greatly from those based on current IPCC methodology. The latter only takes account of N inputs: direct emissions from agricultural soils are taken to be 1.25% of the N applied, while those from natural soils are taken to be 1% of the N deposited from the atmosphere, however, observed values range from 0.2% to 15%. Inadequate accounting for all sources affecting levels of soil mineral N (e.g. freeze-thaw cycles, ploughing, biomass burning, the first rainfall in wet seasons) and inter-annual differences in the size and timing of rainfall events in relation to land management practices are prime causes of the deviations.