Thioglycolic acid and its derivatives as well as various other mercaptocarboxylic acids have become widespread. Both the mercapto and carboxylic functions in thioglycolic acid are chemically active, as is the methylene group in esters under alkaline conditions. The most important reactions involve the thio disulfide interchange reactions with cystine units in hair protein, keratin, which property is applied in cosmetology for hair waving, and the formation of esters to give tin stabilizers for poly(vinyl chloride). For esters, the ability to build N and S heterocycles promises prospective development in fine chemistry. Other mercaptocarboxylic acids have experienced interesting developments, mainly as polymer modifiers and polymer stabilizers.