Carbonaceous aerosol, a major component of particulate matter (PM), gases, and vapors in the atmosphere, has been associated with natural and anthropogenic air pollution, reduced visibility, climate modulation, material and ecosystem damage, and adverse health effects. More recently, epidemiological studies have indicated associations between organic fractions of ambient PM and adverse respiratory and cardiovascular health outcomes. The effects of the non-PM components of the organic aerosol have received less attention because their measurement in the general environment is not mandated. This article summarizes current knowledge of the nature, prevalence, and health effects of organic aerosols encountered in the outdoor environment, identifies key information gaps, and presents a conceptual framework for research priorities for resolving those gaps. The broad, diverse class of air contaminants comprising organic aerosols may be more important to public health than the modest attention given to them. This review focuses on hazard identification and exposure assessment for evaluating risks to public health from ambient organic aerosols. Current knowledge is insufficient to support a quantitative characterization of the aggregate risk from organic air contaminants. Assessments should be done for individual species or mixtures. Efforts should be taken to assemble and evaluate a common set of standard reference materials for both organic speciation and health response assays. A greater standardization of approaches across studies and laboratories would be useful to achieve uniformity in assessing health effects. Multidisciplinary research efforts are needed to improve the current regulatory-driven air quality monitoring networks for epidemiological studies. The limited array of biomarkers linking organic aerosols to health effects needs to be expanded and specific organic compounds or classes that are associated with biological effects in human cells or animal studies need to be tested for better understanding of the exposure-response relationship.