This chapter presents a synthesis of the typical mechanisms and patterns of catalytic redox processes that take place in the natural aquatic environment with the participation of oxygen, its reactive species, hydrogen peroxide, OH radicals, and various forms of copper and iron transition metals. There are presented the redox subsystems (slow or fast) which form the redox character of the environment; the mechanisms and conditions for activating oxygen for its involvement in redox processes; biotic and abiotic sources and pathways of the hydrogen peroxide degradation, as well as its role in the formation, maintenance, and regulation of the environment redox state; and formation and peculiarities of hydroxyl radicals' participation in the processes inside the water basins.