This chapter is intended to provide a detailed weight of evidence scientific review on polychlorinated biphenyls (PCBs) and polybrominated biphenyls (PBBs). Notably, PCBs are among the most thoroughly studied chlorinated persistent organic pollutants to date; despite their discontinued commercial production in the 1970s, their environmental persistence and widespread distribution continue to garner research attention regarding potential impacts on human health. Due to the voluminous literature pertaining to exposure and toxicology of PCBs, for practical purposes, this chapter was written to capture the scientific highlights of greatest import with respect to knowledge on exposures and toxicological properties of these compounds.