Oxidants are added to water and wastewater for disinfection, slime removal, oxidation of undesirable inorganic species (e.g., ferrous iron, reduced manganese, sulfide, and ammonia) and oxidation of organic constituents (e.g., taste- and odor-producing compounds and trihalomethane precursors). Oxidant demand refers to the difference between the added oxidant dose and the residual oxidant concentration measured after a prescribed contact time and at a given pH and temperature. Four methods for determining oxidant demands are given: one method each for chlorine and chlorine dioxide and two methods for ozone (batch and semi-batch methods). An introduction offers recommendations for testing oxidant demand. Suggestions for selecting the proper method to use for determining oxidant demand are also given.