As the Clean Air Act has aged, frustrations have developed with its basic structure. From the vantage point of the year 2000, in fact-and in spite of the success of the Act-these frustrations seem greater than at any point in its past. Nevertheless, with but a few exceptions, the changes made to the act in 1977 and 1990 have never amounted to too much more than tinkering with the basic framework laid out in 1970.