In an attempt to meet the public's needs for information on air quality a variety of indexes have been developed and they continue to evolve. To show the complexity and the diversity of such indices, a variety of current air quality indices are described here and compared in regard to their performance and ability to deliver quality information. A number of characteristics seem desirable for an index: consistency, simplicity, versatility and flexibility. In terms of their ongoing development, an AQI also needs to be useful for forecasting, and the method of calculation needs to be sufficiently flexible to allow for pollutants to be added or subtracted as changes to their health impact are revealed. However, much progress is still to be made, mainly through more careful consideration of the combined impact of multiple pollutants, consideration of low level exposure, and with more timely transfer of usable information to the public.