Disinfection of natural water resources by chlorination to produce potable water results in the production of trihalomethanes. A number of environmental authorities have suggested that these trihalomethanes might have toxic and carcinogenic properties and have an effect on human health. Various monitoring studies have demonstrated appreciable concentrations of trihalomethanes in the effluents of water treatment plants, water distribution systems and domestic tap water. Hence, the majority of households that use tap water as a primary drinking source might be subject to any adverse effects. The possibility of health effects from the unwanted trihalomethanes is minimised by regulating their concentration in water. In order to maintain the standards established by the regulations, various approaches have been developed for controlling the level of these compounds. The object of this paper is to present an overview of chlorine disinfection of water and the formation, health effects and control of trihalomethanes as by products.