Methylmercury (MeHg) is considered a major environmental pollutant that bioaccumulates in fish tissues in direct relation to their age and predatory status. Growing evidence indicates that MeHg toxicity induces a selenium deficiency disease since most of the hallmarks of severe selenium deficiency are also present in mercury toxicity cases. The main source of MeHg exposure in humans is through seafood ingestion. This book presents current research from across the globe in the study of methymercury, including the role of selenium in mitigating mercury toxicity; fish as a dietary source of mercury and methylmercury; as well as in vitro MeHg toxicity studies.