Mercury (Hg) has been used commercially and medically for centuries. Exposure to toxic mercury (Hg) is a growing health hazard throughout the world today. Type and serious of mercury toxicity depends on the form of mercury and exposure way. All chemical forms of mercury can be toxic. In serious toxicity of mercury nervous system, kidneys, respiratory system, immune system, mouth, tooth and periodontal tissues, skin are effected. Diagnosis was made by the help of history, datas and laboratory results. This article reviews the toxicology of mercury, current exposures and clinical manifestations.