Chemical contaminants are widespread in our environment, and we all carry some of many different chemicals in our bodies. What these chemicals do to our health is a very important question. While many chemicals can cause harm to humans, we know much about two categories of persistent toxic substances, metals such as lead and mercury and organohalogens, such as polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs). These substances are very persistent in the environment and in the human body, and can increase risk of a variety of diseases. It is imperative that we find ways of removing these dangerous chemicals from the environment and ways of reducing human exposure.