This chapter discusses various aspects of thallium, an inorganic metallic irritant. Toxic compounds of thallium include thallium acetate and thallium sulphate. In acute poisoning, signs and symptoms start between 12-36 hours to 12 days. In sub-acute cases, there is encephalopathy with white stripes across the nails. In chronic exposure, these symptoms appear in milder forms. In fatal cases, death is preceded by delirium, convulsions and coma. For treatment, patient should be kept warm, Emesis is indicated within 4-6 hours of ingestion, and activated charcoal may be given, followed by saline purgative. Prussian blue or berlin blue which acts to sequester the ions in the intestine and preventing their absorption, is given in a dose of 4 g every 8 hours orally. Postmortem findings in thallium toxicity show anemia and loss of hair and mucous membrane may be inflamed and there may be submucous petechial hemorrhages.