Nitrosoamines, mainly N-nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA), are highly mutagenic compounds that are suspected ofcarcinogenic activity in the human body. The US Environmental Protection Agency (US EPA) classified these compounds into the B2 group, which indicates probable carcinogenicity to humans. According to the US EPA, the maximum admissible concentration of these compounds in drinking water is 7 ng L-1 (NDMA) and 2 ng L-1 (NDEA) with the risk estimation of 10(-5). Until now, knowledge of the formation of nitrosoamines has been based on the mechanism of a secondary alkylamine reaction with nitrite. The problem becomes critical when the concentration of NDMA reaches 10 ng L-1 in surface waters and 20 ng L-1 in drinking water production wells that are under the influence of recharge water from wastewater treatment plants. Furthermore, waters coming from purified sewage were contaminated with 100 ng L-1 of NDMA. It was reported in 2001 that NDMA is formed during the disinfection of water and sewage with chlorine. The results indicated that the reaction of chlorination ofwater containing dimethylamine with chlorine leads to the formation ofNDMA as one of many by-products. NDMA comprises less than 5% of these products. The concentration of NDMA was closely related to the ratio of chlorine, ammonia ions and dimethylamine. Furthermore, the concentration of nitrosoamine increased with the concentration of monochloramine as well as the reaction time. It should be emphasized that the maximum concentration of NDMA is formed at pH 7-8. Therefore, NDMA can be treated as a new disinfection by-product such as THMs, HAAs or MX. Experimental results indicated that the usage of both chlorine and chlorine dioxide leads to the formation of NDMA. In model experiments, in the aqueous solutions containing dimethylamine and ammonia ions disinfected with chlorine dioxide, a few hundred micrograms of N- dimethyinitrosoamine were found. Contrary to the reactions of dimethylamine with chlorine and chlorine dioxide, reactions with ozone do not lead to the formation of NDMA.