The paper provides an overview of the sources, the estimation methodology, and the relative amounts of natural hydrocarbon and nitrogen oxide emissions. The most recent estimate of natural nonmethane hydrocarbon (NMHC) emissions for the United States is 28 teragrams per year (Tg/yr). This compares with 20 Tg/yr for anthropogenic NMHC sources. The southeastern and southcentral portions of the United States account for approximately 43% of the annual U.S. natural NMHC estimate. These emissions exhibit strong diurnal and seasonal dependencies related to temperature, solar radiation, and active biomass. Forests are the primary vegetative source of hydrocarbons. The major sources of natural nitrogen oxide emissions in North America are biomass burning, lightning, and microbial activity in soil. The authors present a comparison of hourly gridded nitrogen oxide emissions from lightning, soil, and man-made sources for the northeastern United States. They also provide results from preliminary investigations of the sensitivity of ozone predictions from the U.S. Environmental Protection Agency's Regional Oxidant Model to natural NMHC and nitric oxide emissions.