Homocysteine thiolactone is a five-membered cyclic thioester of amino acid homocysteine. It is generated from homocysteine as a result of an error-editing reaction, principally, of methionyl-tRNA synthetase. An elevated level of homocysteine thiolactone is associated with cardiovascular diseases, strokes, atherosclerosis, neurological abnormalities, etc., presumably because it reacts to the side chain of protein lysine causing protein damage and autoimmune responses. It is not only an important metabolite but also a versatile building block for organic and bioorganic synthesis. This entry contains data on the homocysteine thiolactone formation, metabolism, toxicity mechanism in vivo, and the bioorganic chemistry applications as a powerful synthetic tool in polymer science, sustainable materials development, and probes.