Haloacetic acids (HAAs) are a second major class of disinfection by-products (DBPs) arising from the chlorination of drinking water. They have not been studied as extensively as the trihalomethanes (THMs), but in many waters they are found at concentrations equal to, or greater than, the concentration of THMs. Health effects research suggests that some HAA species are more harmful than THM species. This paper summarizes a number of the author's recent investigations concerning the formation, occurrence, stability, and control of HAAs in chlorinated drinking water. A number of examples linking observations made under controlled laboratory conditions to field-scale observations are presented, and the significance of the findings to water treatment practice is discussed.