This overview is based on experimental and epidemiological studies of NOx toxicity during the past decade. Approximately 130 published studies are cited and about one-fourth of these are discussed briefly under one of the following headings: acute and subacute studies, chronic low-level studies, human studies, and special studies. The latter section examines a selection of comparatively unique investigations, including several devoted to the pulmonary uptake and retention of NO2, and several examining the potential tumorigenicity of NO2. For each major section of the overview, a critical evaluation is attempted in terms of the impact of the appropriate studies on the extant NOx toxicological data base and on the current and planned air quality standards for NOx.