IMPLEMENTATION ANALYSIS AND ASSESSMENT**The chapter is revised from an earlier paper of the same title that appeared in Policy Analysis, Volume 1, No. 3, pp. 531–566 (Copyright by The Regents of the University of California), and the portions used from the earlier work are included with the permission of The Regents of the University of California. Work on the paper was supported by a grant from the National Science Foundation. A number of individuals commented on earlier drafts: Richard Elmore, Lucille Fuller, Eleanor Holmes, Laura Kemp, and Jeanette Veasey, all at the time at the University of Washington; Robert Levine, Congressional Budget Office, and Arnold J. Meltsner, University of California, Berkeley. The author, however, is solely responsible for the views expressed.