Drinking water safety cannot be taken for granted because there are many chemical, biological, and physical threats to drinking water supplies. Chemical and biological threats include contamination from improper chemical handling and disposal, animal wastes, pesticides, human wastes, stormwater runoff, and naturally occurring substances. Drinking water that is not properly treated or disinfected or that travels through an improperly maintained distribution system can also become contaminated and pose a health risk. Physical threats include failing water supply infrastructure and threats posed by tampering or terrorist activity. This chapter provides an overview of drinking water treatment processes for surface and groundwater through a discussion of conventional treatment systems, advanced treatment processes for organic and inorganic chemicals of concern, treatment process by-products and related health concerns for these materials, and concerns regarding water quality changes in water distribution systems. An applications section provides five Illustrative Examples related to the general subject of drinking water treatment.