This book evaluates the risks to human health and the environment posed by exposure to polybrominated dibenzo-p-dioxins (PBDDs) and polybrominated dibenzofurans (PBDFs). Of no commercial use, these compounds are generated as unwanted by-products in various industrial and combustion processes and have been detected as contaminants in a number of brominated organic chemicals, many of which are used as flame retardants. Thermolysis of brominated flame-retardants is an important source of emissions, as is the incineration of products containing these flame-retardants, most notably scrap computers and business machines. PBDDs and PBDFs have also been detected in emissions of motors using both leaded petrol and unleaded petrol, with and without catalytic converters, and in emissions of diesel engines. In view of the complexity of these compounds, the problems with analytical procedures, and substantial gaps in the experimental database, the report makes a special effort to determine the extent to which the environmental behaviour and toxic effects of PBDDs and PBDFs resemble those of their better characterized chlorinated analogues, polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). An effort is also made to determine the extent to which PBDDs and PBDFs contribute to the overall hazard posed by environmental dioxins.