Polybrominated dibenzo-p-dioxins (PBDDs) and polybrominated dibenzofurans (PBDFs) are almost planar tricyclic aromatic compounds. Theoretically, 75 PBDDs and 135 PBDFs are possible. In addition, a large number of mixed halogenated congeners 1550 brominatedlchlorinated dibenzo-p-dioxins (PXDDs) and 3050 brominatedlchlorinated dibenzofurans (PXDFs) are theoretically possible. Because of the complexity of the analytical procedures and paucity of analytical reference standards, it has been possible to characterize and determine only a small number of these compounds. The most toxic congeners are those substituted at positions 2, 3, 7, and 8. There are 7 2,3,7,8-substituted PBDDs and 10 2,3,7,8-substituted PBDFs, as well as 337 possible 2,3,7,8-substituted PXDDs and 647 possible 2,3,7,8-substituted PXDFs.