Plasmas generated by electrical discharges in liquids and gas–liquid environments have been studied for many years for pulsed power applications and high-voltage insulation, and increasing interest has been shown to applications in water pollution control, nanoscience, sterilization, and plasma medicine. This entry addresses mainly the application of plasma in liquid and at the gas–liquid interface for water treatment. The mechanism of discharge formation directly in the liquid phase is significantly different from that in gas, and therefore, the initiation of electrical breakdown in liquid is discussed, as well as streamer propagation and properties. A large number of electrode geometries have been reported, and the most important are described, with emphasis on discharges that enhance plasma volume, minimize electrode erosion, and improve the efficiency for water decontamination. In addition, the generation of chemically active species in discharges in liquid and in contact with liquid is also addressed. The most abundant species responsible for the oxidation of organic matter are the hydroxyl radical, ozone, hydrogen peroxide, and peroxynitrite. Their formation and chemical reactions with organic compounds are briefly discussed. The degradation of aqueous pollutants by nonthermal plasma is reviewed. The chemical compounds most commonly used were selected: phenols, organic dyes, pharmaceutical compounds and personal care products, and pesticides. The removal of the target compounds is reviewed, and the degradation intermediates and final products and the degradation mechanism under plasma conditions are mentioned, where available.