Water chemistry is important because several factors about water to be treated and then distributed or returned to the environment are determined through simple chemical analysis. Probably the most important determination that the water practitioner makes about water is its hardness. This chapter discusses some of the more common constituents of water along with the parameters of alkalinity, water temperature, specific conductance, and hardness of water. In order to operate a water treatment process correctly and safely, water operators need to know the types of chemicals used in the processes, what the purpose of each is, and the safety precautions required in the use of each. The chapter briefly discusses some water treatment chemicals. It reviews general chemical parameter categories such as dissolved oxygen organics (biochemical oxygen demand and chemical oxygen demand), synthetic organic chemicals, volatile organic chemicals, total dissolved solids, fluorides, metals, and nutrients—the major chemical parameters of concern.