During my last year as coeditor of Water Resources Research, I undertook a project which culminated in this volume, the first comprehensive special issue in the 22-year life of the journal. The decision to prepare a special issue of invited papers resulted from discussions with a number of individuals and my assessment that a need existed to provide greater coherence to the study of hydrology. There have been other attempts in the last decade to place specific aspects of hydrology into perspective. One notable effort was by Myron Fiering, who asked colleagues to write on the scientific basis of water resource management [U.S. National Research Council, 1982]. While the papers contained in the current volume do not have as extensive a subject base as those in the earlier work, they do address many important topics in hydrology. The 19 authors who have contributed to the 15 papers in this volume have, collectively, over 500 years of professional experience.