This Guideline addresses the three classes of PCBs, PCTs and PBBs. PCBs represent by far the most significant group in terms of quantities originally produced, the volumes of waste requiring disposal, and the environmental significance of past, present and future waste disposal practices. PCBs have been studied much more than PCTs and PBBs, and hence the information available relates mainly to PCBs. In may respects, the three groups of substances display similar characteristics, and can cause similar problems if not handled properly.