This paper is one of nine papers that examine issues related to drinking water distribution systems. The purpose of this document is to review existing literature, research, and information on the occurrence, magnitude, and nature of the public health risks associated with cross-connections and backflow, from both acute and chronic exposures, and methods for detecting and controlling the occurrence of cross-connections and backflow within distribution systems. More specifically, the goal of this document is to review what we know regarding: (1) causes of contamination through cross-connections; (2) the magnitude of risk associated with cross-connections and backflow; (3) costs of backflow contamination incidents; (4) other problems associated with backflow incidents; (5) suitable measures for preventing and correcting problems caused by cross-connections and backflow; (6) possible indicators of a backflow incident; and (7) research opportunities.