The efficacy of chlorination to prevent and check epidemics of water-borne typhoid has never been doubted. Innumerable instances could be cited in which the prompt treatment of large public supplies has promptly checked outbreaks that threatened to assume serious proportions and there is no doubt that the extremely low typhoid morbidity rate on the Western Front of the European battlefield is partially due to the extensive and rigorous chlorination measures that have been instigated. Prophylactic vaccination and the prompt isolation of typhoid carriers have largely contributed to the wonderful results obtained but due credit must also be given to the systematic purification and treatment of water supplies. Similar results have been obtained at training camps in Canada and in other countries by effective treatment with either liquid chlorine or hypochlorite. Since the inception of water chlorination in America in 1908, the merit of the method has been very generally recognized throughout the Continent but was regarded with scepticism in Europe, except as a temporary expedient, until the results obtained by the military forces compelled more general recognition. Before the war, chlorination of water supplies in England was only practised in a few isolated and relatively unimportant instances; in 1917, practically the whole supply of London was chlorinated and at Worcester a similar treatment has been recommended to enable the slow sand filters to be operated at higher rates without reducing the quality of the water supplied to the consumers.