Chloral is a chlorinated aldehyde that found extensive use, beginning in the 1940s, as a precursor in the production of the insecticide DDT and, to a lesser extent, of other insecticides and pharmaceuticals. This use of chloral has declined steadily since the 1960s, especially in those countries where the use of DDT has been restricted. Chloral is readily converted to chloral hydrate in the presence of water. Chloral hydrate is used as a sedative before medical procedures and to reduce anxiety related to withdrawal from drugs. Wider exposure to chloral hydrate occurs at microgram-per-liter levels in drinking water and swimming pools as a result of chlorination. Chloral hydrate is a well-established aneuploidogenic agent that also has some mutagenic activity. In human cells in vitro, chloral hydrate induced aneuploidy, micronuclei and gene mutations. Chloral hydrate clearly induced micronuclei in Chinese hamster cells, whereas findings in mouse lymphoma cells were conflicting. Induction of somatic mutation (but not sex-linked mutation) by chloral hydrate was demonstrated in insects. Chloral hydrate is metabolized in vivo to trichloroethanol, which is responsible for its physiological and psychological effects. The metabolite of chloral hydrate exerts its pharmacological properties via enhancing the GABA receptor complex and therefore is similar in action to benzodiazepines, nonbenzodiazepines, and barbiturates. In clinical studies, oral chloral hydrate appears to have a lower sedation failure rate when compared with oral promethazine for children undergoing pediatric neurodiagnostic procedures. The sedation failure was similar for other comparisons such as oral dexmedetomidine, oral hydroxyzine hydrochloride, and oral midazolam. When compared with intravenous pentobarbital and music therapy, oral chloral hydrate had a higher sedation failure rate. Compared to dexmedetomidine, chloral hydrate was associated with a higher risk of nausea and vomiting.