Biocides are essential chemical substances used in water treatment to control and eliminate harmful microorganisms that can cause biofouling, corrosion, and health risks. There are two main categories of biocides: oxidizing biocides, which work by oxidizing cell membranes, and non-oxidizing biocides, which target specific cellular processes or structures. Biocides are used in various water treatment applications, such as cooling water systems, reverse osmosis, ultrafiltration, wastewater treatment, oilfield water treatment, and the pulp and paper industry. When selecting a biocide, factors such as the type of microorganisms present, water chemistry, temperature, pH, and compatibility with other water treatment chemicals should be considered. Proper handling and dosing procedures are essential, as biocides can be toxic to humans and the environment if misused, and their use is governed by regulations and guidelines set by agencies like the U.S. EPA and REACH in the European Union.