This publication addresses the urgency of controlling pollution and preserving water quality around the world. The past few decades have focused on the importance of water quantity in meeting basic human and ecosystem needs for water. Water quality is as important as water quantity for satisfying human and environmental needs, and yet has received far less investment, scientific support, and public attention. An overview of water-quality challenges details the contaminants and human activities that affect water quality. Water quality impacts human health, water quantity, livelihood, and economic activity, and climate change. Emerging contaminants, population growth and urbanization also present additional stresses to water quality.