Mercury (Hg) is a global pollutant, and methylmercury (MeHg) toxin is produced predominantly by a small group of anaerobic microorganisms possessing two key genes (hgcA and hgcB) necessary for converting inorganic Hg to methylmercury, which bioaccumulates and biomagnifies at high levels in the food web. However, net methylmercury production in the environment also depends on a reverse process, namely demethylation, catalyzed by both chemical-photochemical and biological reactions. This presentation will provide a perspective of our current understanding of the biochemical mechanisms of Hg methylation by microorganisms, coupled biotic and abiotic interactions and environmental factors controlling Hg and methylmercury transformations in natural ecosystems. I will also discuss future research needs at further elucidating these fundamental processes and pathways governing Hg and methylmercury transformation and behavior, thereby facilitating targeted strategies to minimize the adverse impacts of methylmercury in the natural environment.