Mercury occurs naturally in the environment and is released by human activities. Mercury exists in gaseous, liquid, and solid phases, and all phases are of importance when fate s effects of mercury in terrestrial, fresh and marine water, and atmospheric environments. Mercury can be transformed to a highly toxic form of methylmercury. Humans are exposed to the toxicity of methylmercury by eating fish. Methylmercury is bioaccumulated up the food chain by transfer of residues of methylmercury in smaller organisms that are food for larger organisms in the chain. This sequence of process results in higher concentrations in organisms at higher levels in the food chain with human at the top of the food chain. This study is an evaluation of chemical speciation and biological processes that govern mercury distribution and transformation among three environmental media: atmosphere, water, and sediments. Understanding speciation of mercury and biological processes of methylmercury transformation plays an important part in toxicity and exposure of mercury to living organisms. Speciation also influences transport of mercury within and between environmental media while biological processes of methylmercury transformation influence methylmercury production and its transport to the biological communities. Study also covers the demethylation process that can convert methylmercury to inorganic mercury species. Demethylation and methylation processes therefore may occur in parallel further complicating the assessment of mercury fate in the environment. The study will provide integrated fundamental pathways of mercury species transformation through chemical and biological pathways and will contribute to an understanding of fate and transport of mercury species in environmental media. It will also provide a foundation for a state- and region-wide examination of mercury monitoring and control strategies.