Monomethylmercury (MMHg) concentrations in marine stratus cloud and fog water in California are enhanced to upwards of 1 ng L-1 compared to concentrations typically found in rain water, which are closer 0.1 ng L-1. This raises the possibility that coastal terrestrial ecosystems impacted by marine fog and clouds could be exposed to this potent neurotoxin. Efforts to measure MMHg in cloud and fog water on the California coast indicate that the source of MMHg is from evasion of dimethylmercury (DMHg) and subsequent demethylation in the air or in the droplet to form MMHg.  This presentation will highlight the data collected on land, at sea, and by aircraft at numerous locations in coastal California since 2014, and discuss the major gaps in our understanding of the cycling and transport of methylated Hg compounds in the oceanic mixed layer, atmospheric marine boundary layer, and the cloud droplet.