Extreme toxicity of some species of mercury, the ability of this element to bioaccumulate in particular in fish meat, and the known cases of lethal poisoning by mercury have drawn particular attention to this element's presence in the natural environment. Due to the relatively long time of its presence in the air, elemental mercury can be transported over large distances, hence the presence of mercury of anthropogenic origin is detected practically all over the world. Apart from the elemental mercury, the main species of mercury in water are Hg(II) and mercury-organic species, in particular methylmercury. The latter undergoes strong bioaccumulation in living organisms and concentration in the trophic chains. That is why the relative concentration of mercury in organisms is determined by its presence in water. The concentration of mercury in water is related to the processes of methylation and demethylation, influenced by biotic and abiotic factors such as the activity of microogranisms, access to oxygen, illumination, temperature, pH and others. Despite intense studies, full and reliable recognition of the ecological and health effects of pollution by this toxic metal is still impossible. The aim of this paper is to present the problems related to speciation of mercury, and describe some conversion and migration processes of mercury in the water environment.