NOx is emitted into the atmosphere from natural and anthropogenic sources, i.e., from fossil fuel combustion, biomass burning, oxidation of atmospheric ammonia, and lightning. Transport of NOx from the stratosphere and aircraft emissions can also be import ant sources in the upper troposphere. Among the various NOx sources, the contribution from lightning likely represents the largest uncertainty. We evaluate the impact of anthropogenic and natural NOx sources on tropospheric NOx and O3 levels by using a global 3D chemical transport model. The effects of major surface NOx emission sources (including anthropogenic, biomass burning, and soil emissions) are compared with that of lightning-produced NOx. The results reveal that a thorough assessment of atmospheric NOx emission sources and their impact is required to devise control strategies for regional and global air pollution.