Advanced oxidation processes (AOP) are processes that are used for the purpose of breaking down biochemical or chemical persistent organic components in waste water with a certain consumption of energy. They are used as a substitute for biological water purification processes when they are not effective in removing certain organic components. Their main role is the mineralization of pollutants disinfection of water so that it is safe to return to the environment or to drink. The decomposition of organic components takes place with the expenditure of energy, which is necessary for the formation of reactive intermediates. Advanced oxidation processes are divided into three groups according to energy consumption. The low energy consumption group includes processes like O3, O3/H2O2, O3/UV, UV/H2O2, UV/S2O82- and UV/Cl2. The second group requires medium energy consumption and includes photo-Fenton, plasma and electrolytic advanced oxidation processes. The third group requires high energy consumption and includes UV-photocatalysis and processes based on microwaves and ultrasound. This paper provides an overview of an advanced oxidation process from the first group of processes that require low energy consumption, the UV/Cl2 process. Its effectiveness is based on the formation of HO● (hydroxyl radicals) and Cl●, ClO● and Cl2●−, which have the common name of "reactive chlorine species". The UV/Cl2 advanced oxidation process has its advantages and disadvantages, and as a process it is relatively new, which is why it is working on its development to make it as efficient and environmentally friendly as possible.