Mercury exists in various forms, and people are exposed to each in different ways. Mercury is used in thermometers, barometers, manometers, sphygmomanometers, other devices. Mercury is an ingredient in dental amalgams .Some medicines use small amounts of these compounds as preservatives e.g. thimerosal.Mercury poisoning can result from exposure to water-soluble forms of mercury (such as mercuric chloride or methylmercury present in fishes), by inhalation of mercury vapour, or by ingesting any form of mercury. The most toxic forms of mercury are its organic compounds, such as dimethylmercury and methylmercury. Other exposures may result from using or breaking products containing mercury. Mercury can cause both chronic and acute poisoning. A severe form of mercury poisoning first presented in Japan as Minamata disease. Dental amalgam is a direct filling material used in restoring teeth. It is made up of approximately 40-50% mercury. Metallic mercury when used as dental filling, mainly causes health effects when inhaled as a vapour where it can be absorbed through the lungs. Symptoms of prolonged and/or acute exposures include: tremors, emotional changes, insomnia, neuromuscular changes, and disturbances in sensations. In Conclusion, Mercury and most of its compounds are extremely toxic and must be handled with care.