Nearly 3700 organohalogen compounds, mainly containing chlorine or bromine but a few with iodine and fluorine, are produced by living organisms or are formed during natural abiogenic processes, such as volcanoes, forest fires, and other geothermal processes. The oceans are the single largest source of biogenic organohalogens, which are biosynthesized by myriad seaweeds, sponges, corals, tunicates, bacteria, and other marine life. Terrestrial plants, fungi, lichen, bacteria, insects, some higher animals, and even humans also account for a diverse collection of organohalogens. The incredibly large number of marine and terrestrial organisms that are awaiting exploration for their chemical content virtually guarantees the discovery of numerous new natural organohalogens, many of which will doubtless have significant biological activity. It is inevitable that additional mammalian organohalogens will be identified and their role in the biodisinfection process understood. The clear and convincing evidence that chlorinated dioxins and dibenzofurans have several natural sources (both abiogenic and biogenic) is one of the most significant and politically important scientific discoveries of our age. Includes 89 references, figures.