The multiple barrier concept is applied by public health professionals as they attempt to prevent waterborne transmission of communicable diseases. This chapter discusses two water treatment techniques, filtration and disinfection, that are commonly used to provide barriers to disease transmission. The portion of this work on filtration includes information on rapid rate granular media filtration, slow sand filtration, and diatomaceous earth filtration. Capabilities of these processes to reduce concentrations of viruses, bacteria, and cysts are presented. Design and operational factors that influence filter performance are discussed. General considerations related to disinfection are given, after which the microbial inactivation capabilities of chlorine species (free and combined), chlorine dioxide, and ozone are discussed. This chapter contains information on inactivation of a variety of microorganisms, including coliforms, Giardia cysts, and poliovirus.