This cluster of papers focuses on the occurrence, genotoxicity, and carcinogenicity of disinfection by-products (DBPs) in drinking water, with a specific emphasis on the effects of chlorination and alternative disinfectants. It also explores the formation, control, and health implications of various DBPs, including nitrosamines, haloacetic acids, and halonitromethanes. The cluster delves into the microbial ecology of drinking water distribution systems and the characterization of natural organic matter in relation to DBP formation.