This book is the planned successor to A Geologic Time Scale by W. B. Harl&, A. V. Cox, P. G. Llewellyn, C. A. G. Pickton, A. G. Smith & R. Walters published in 1982. It adopts the same style & employs & develops similar methods, but it has been entirely reworked. The state of the art 1989 is thus presented; but the data assembled provide a source of reference which will serve for some years. The work develops & assesses a new calibration of the geologic time scale employing a new database. The new scale is summarized on the book covers & on a wall chart to be published separately. The work has been coordinated in detail throughout; nevertheless each chapter can stand on its own.