Droughts, floods and pollution are frequently viewed as constraints to economic and social development. How too little, too much or over-polluted water is managed can determine the extent to which this critical resource contributes to human welfare. How can those managing our water resources do so in a way that meets society’s changing objectives and needs? This publication considers how water resources can become more integrated and sustainable. It introduces the science and art of modeling in support of water resources planning and management. The authors draw on their extensive experience to provide a variety of management tools that can be used in water resources system planning, development and management projects worldwide.