This chapter discusses the properties and use of haloperoxidases in organic synthesis. Haloperoxidases are enzymes that are capable of halogenating a variety of organic compounds using hydrogen peroxide and halide ions as substrates. Haloperoxidases are known from almost 100 sources, including mammals, birds, plants, algae, molds, and bacteria. The main sources of haloperoxidases can be divided into two groups: algae and bacteria. Algae produce several heme-containing haloperoxidases. Although all the heme-containing haloperoxidases that have been isolated so far possess ferriprotoporphyrin IX as the prosthetic group, not all enzymes have the same halogenating abilities: some can chlorinate, while others can only brominate. Moreover, the prospects for haloperoxidase research are still good, particularly in the field of nonheme microbial chloroperoxidases, which are stable toward heat and oxidizing chemicals. It is suggested that once large-scale production of these enzymes is advanced, their applications in industrial organic synthesis can follow immediately. © 1992, Academic Press Inc.