Per- and polyfluoroalkyl substances (PFAS) are a family of over 5,000 man-made chemicals with many useful properties including the ability to repel water, prevent staining and increase heat resistance. PFAS have many industrial and consumer uses including the coating of fabrics, carpets, electrical wire, and non-stick cookware, in food packaging (e.g., microwave popcorn bags and fast-food wrappers), as a mist suppressant in metal plating, and in firefighting foam used by firefighters to put out petroleum fires, but not typically in home fire extinguishers. Four of the most studied PFAS, perfluorooctanoic acid (PFOA), perfluorooctane sulfonic acid (PFOS), perfluorononanoic acid (PFNA) and perfluorohexane sulfonic acid (PFHxS), are found with the highest frequencies and concentrations in the environment, in humans and in wildlife. We know the most about the harmful effects and environmental fate of these four PFAS. While PFOS and PFOA have been phased out of production, they are very persistent chemicals and can remain in the environment for long periods after being removed from the marketplace.