Polychlorinated biphenyls (PCBs) were manufactured and used widely for many years. Because they are very persistent in both the environment and biological systems, there has been significant global contamination. This review presents a summary of known or suspected health effects of various PCB congeners, documented on the basis of both human and animal studies. As our knowledge increases several important points become apparent. PCBs interfere with many biological functions, including the immune system, the nervous system, and several endocrine system, and the fetus appears to be particularly vulnerable to these actions. PCBs cause certain cancers in animals. PCBs are mixtures of multiple congeners, differing on the basis of the numbers and positions of chlorines around the biphenyl ring, and it is becoming increasingly apparent that different congeners may have very different actions. These observations suggest that the potential human health hazards from PCB exposure have been underappreciated.