Experts are increasingly concerned that exposure to toxic substances in our environment is impairing our reproductive and developmental processes. Biologic markers have emerged as a promising tool for understanding the environmental effects of toxic substances. This report clarifies the underlying concepts of the use of biologic markers in general and explores how a broad range of biologic markers may be the key to our understanding of environmental health effects, particularly in the reproductive field, early enough to make a difference in people's lives. The four major text sections in this book are male reproduction, female reproduction, biologic markers of pregnancy, and biologic markers of neurodevelopment. Each section presents a detailed view of what is understood about human systems--including the most recent research results--and how biologic markers may specifically be applied.