The Lower Rio Grande Valley Transboundary Air Pollution Project (TAPP) was a U.S.-Mexico Border XXI Program project to assess transboundary air pollution in and near Brownsville, Texas. The study used a three-site air monitoring network very close to the border to capture the direct impact of local sources and transboundary transport. Ambient data included particulate mass and elemental composition, VOCs, PAHs, pesticides, and meteorology. Also, near real-time PM2.5 mass measurements captured potential pollutant plume events occurring over 1-h periods. Data collected were compared to screening levels and pollutant plume events occurring over 1-h periods. Data collected were compared to screening levels and other monitoring data to assess general air pollution impacts on nearby border communities. Wind sector analyses, chemical tracer analyses, principal component analyses, and other techniques were used to assess the extent of transboundary transport of air pollutants and identify possible transboundary air pollution sources. Overall, ambient levels were comparable to or lower than other urban and rural areas in Texas and elsewhere. Movement of air pollution across the border did not appear to cause noticeable deterioration of air quality on the U.S. side of the Lower Rio Grande Valley. Dominant southwesterly winds from the Gulf of Mexico were largely responsible for the clean air conditions in the Brownsville airshed. Few observations of pollutants exceeded effects screening levels, almost all being VOCs; these appeared to be due to local events and immediate influences, not regional phenomena or persistent transboundary plumes.