Water quality is essential to the welfare of humans and aquatic ecosystems. The study of water quality draws information from a variety of disciplines including chemistry, biology, mathematics, physics, engineering, and resource management. University training in water quality often is limited to specialized courses in engineering, ecology, and fisheries curricula. Therefore, many professionals who need a basic understanding of water quality are not formally trained in the subject. This text provides an in-depth but relatively simple treatment of water quality, including a discussion of basic physical, chemical, and biological principles. Effort has been made to use physical and chemical principles to explain the factors controlling the quality of natural waters. This work is a text for a general course in water quality or as a guide for self-study.