Mercury (Hg) is mobilized from the Earth’s crust and accumulates as monomethylmercury (CH₃Hg-X, where X is e.g. Cl^- or R-S^-) in aquatic food webs, thereby posing a threat to human and wildlife health. This has motivated over 50 years of research on Hg biogeochemistry. Still, our understanding of the processes controlling the amount of monomethylmercury available for bioaccumulation remains limited. In this talk, I will review recent advances in our mechanistic understanding of formation and degradation of methylated species in natural environments. In addition, I will discuss the role of chemical Hg speciation for Hg methylation and demethylation. Special attention will be given to our understanding of Hg biogeochemistry in coastal and marine systems. The understanding of Hg cycling in marine environments is particularly incomplete even though marine harvested food is the primary monomethylmercury exposure route for most human populations. A unique feature of marine systems compared to freshwater systems is that ~30-80% of the total methylated Hg pool occurs as dimethylmercury ((CH₃)₂Hg). The vertical and geographical distribution of mono- and dimethylmercury have been documented in many coastal and oceanographic systems since the 90´s. More recent work has also quantified in situ transformation rates between chemical Hg forms (e.g. methylation and demethylation rates of monomethylmercury). However, what role in situ methylation and demethylation of mono- and dimethylmercury plays for the amount of monomethylmercury available for bioaccumulation is unknown. Finally, I will discuss critical questions remaining to be addressed and constraints with experimentally based studies conducted.