Haloalkane dehalogenases (HLDs) convert halogenated compounds to corresponding alcohols, halides, and protons. They belong to alpha/beta-hydrolases, and their principal catalytic mechanism is S(N)2 nucleophilic substitution followed by the addition of water. Since HLDs generally have broad and different substrate specificities, they have various biotechnological applications. HLDs have previously been believed to be present only in bacterial strains that utilize xenobiotic halogenated compounds, and three archetypal HLDs, i.e., DhlA, DhaA, and LinB, have been intensively investigated by biochemical, structural, and computational analyses. Furthermore, by using the resulting data and target-selected random mutagenesis approaches, these HLDs have been successfully engineered to improve their substrate specificities and activities. In addition, important insights into protein evolution have been obtained by studying these HLDs. At the same time, the genome and metagenome information has revealed that HLD homologues are widely distributed in many bacterial strains, including ones that have not been reported to degrade halogenated compounds. Some of these cryptic HLD homologues have been experimentally confirmed to be "true" HLDs with unique substrate specificities and enantioselectivities. Although their biological functions and physiological roles remain mysterious, these potential HLDs are considered promising materials for the development of new biocatalysts.