A xenobiotic is a chemical which is found in an organism but which is not normally produced or expected to be present in it. Specifically, drugs such as antibiotics are xenobiotics in humans because the human body does not produce them itself. In this book, the authors present current research on xenobiotics including the potential effects of toxic agents on immune system function; medical strategies to prevent adverse effects of PAH-like anthropogenic xenobiotics; organophosphorus pesticides as dangerous xenobiotics for aquatic environments and phytoremediation for removal of organic xenobiotics from water.