This revised, updated textbook presents a systems approach to the planning, management, and operation of water resources infrastructure. Previously published in 2005 by UNESCO and Deltares, this new edition—with contributions from Jery R. Stedinger, Jozef P. M. Dijkman, and Monique T. Villars—introduces viewing water resources issues as interacting systems across components and scales. It offers guidelines for initiating and carrying out planning and management projects and introduces optimization, simulation, and statistical methods for project identification, design, siting, operation, and evaluation. Coverage includes basin-wide and urban water issues, multiple-purpose and multi-objective quantity and quality management, case studies, exercises, and media supplements. The book is open access under a CC BY-NC 4.0 license.