 Mercury exists naturally and as a man-made contaminant with exposure to mercury or mercury compounds resulting in various toxic effects on the nervous system, immune system, cardiovascular system, hematology, pulmonary system, renal system, endocrine system, reproductive system, and embryonic development. Ingested mercury may undergo bioaccumulation leading to progressive increases in body burdens. This article was authored by Kevin M. Rice, Ernest M. Walker, Meowzong Wu, and others.