Water chemistry deals with the fundamental chemical properties of water itself, the chemical properties of other constituents that dissolve in water, and the countless chemical reactions that take place in water. Water is not passive in these chemical reactions. Instead, it plays an active role, constantly making and breaking chemical bonds, thereby facilitating chemical change. This chapter introduces the reader to water chemistry principles including the chemical properties of water, the chemical composition of natural waters, chemical reactions, and an application section that provides six Illustrative Examples related to the general subject of water chemistry.