This paper reviews relevant research findings for the purpose of establishing a set of uniform national criteria for designating locations of air monitoring stations. Data first are presented showing the difficulty, in the absence of uniform criteria, of interpreting measurements from one station in a city to determine if air quality for that city is 'better' or 'worse' than that for another city. Then, research data are given showing, for example, that proximity to traffic (in both the horizontal and vertical directions) affects measured carbon monoxide concentrations. Finally, a scheme is presented for classifying all monitoring stations into six different siting categories. Type A stations are designed to provide a realistic measure of pedestrian exposures, while Type B measures the background concentration extending over a large physical area. Type C stations monitor residential or suburban population exposures. Types D, E, and F are for more specialized purposes, such as collecting meteorological information and conducting air quality surveys. Types A, B, and C, which are most important in implementing air quality standards, must have a uniform probe height of 3 + or - 1/2 meters. A standardized system of site selection, such as the one proposed here, should greatly improve the comparability and meaningfulness of data obtained from different air monitoring stations throughout the Nation.