An extensive survey was conducted of all the air pollution indices that are presently utilized or are available. The data were obtained from a literature review; from telephone discussions with personnel in State, local, and Provincial air pollution control agencies; and from material received from these agencies. Of the 55 metropolitan air pollution control agencies surveyed in the United States, 35 used some form of daily air pollution index. These indices were so varied that it was necessary to develop a system to classify indices according to four criteria: (1) number of variables, (2) calculation method, (3) calculation mode, and (4) descriptor categories reported with the index. Using the classification system, 14 basically different index types were identified. With two minor exceptions, it was found that no two indices were exactly the same. The survey results and agency comments were used to identify the general structural characteristics and criteria for a candidate uniform air pollution index.