The 19th edition of the Dewey Decimal Classification was published in 1979 and will be the last edition to appear under the editorship of Benjamin A. Custer, the editor since the 16th edition, published in 1958. There is thus a suitable opportunity to review the evolution of the Classification over the past thirty years, and to indicate the likely developments in the 20th edition and beyond. This paper is not a literature review although reference will be made to pertinent contributions in the literature. There is a substantial body of literature on the Dewey Decimal Classification and the most recent portion is covered in two bibliographies.