This book actually is about an old problem: pollution. However, it is about a subtle kind of pollution that can enter organisms and alter biological processes without being acutely toxic. Endocrine-disrupting chemicals (EDCs), sometimes in minute quantities, can mimic or inhibit the actions of internal messengers called hormones. As such, many consider EDCs to represent a potential threat to the health of wildlife and humans. The purpose of this book is to bring together in one place the pertinent information on endocrine physiology, toxicological methods, and descriptions of potential and known EDCs to help identify reasonable biomarkers for determining the risks of EDC contamination to wildlife and humans. Although the focus is biased toward vertebrates, the potential impact of EDCs on invertebrate species that form the nutritional base for vertebrate populations cannot be ignored. The principles discussed apply equally well to both vertebrate and invertebrate systems.