The present article discusses the negative impact of municipal solid waste (MSW) landfills on the environment due to the migration of leachate formed during waste decomposition. The leachate contains an extensive list of pollutants that migrates in soils, surface and underground waters. The article provides a review of literature regarding the main stages of leachate formation and changes in its chemical composition during the landfill’s life cycle. Landfill waste undergo biogeochemical decomposition in aerobic and anaerobic conditions. At the initial stages of waste degradation, easily decomposable organic compounds oxidize leading to high values of chemical and biological oxygen demand (COD and BOD) in the formed leachate, lowering the pH of the medium to 4-5. This in turn leads to the transition of metal ions to the liquid phase due to an increase of their activity in acidic mediums. In anaerobic conditions, further waste degradation takes place producing carbon dioxide, methane, ammonia, mercaptans, hydro