The global prevalence of neurodevelopmental disorders is accelerating. Numbers of children affected by an autism spectrum disorder (ASD) in the United States have reached 1 in 88 — 1 in 56 among boys — and even more children have developed attention deficit/hyperactivity disorders (ADHD). The burden of these disorders to individuals and society overall is enormous; ASD alone costs the United States a staggering $130 billion, with ADHD costs reaching similar heights. Genetic causes of these neurodevelopmental disorders cannot account for such radically increased rates of incidence. The causes must also implicate environmental chemicals, many of which have been shown to disrupt normal thyroid function. In this book, Barbara Demeneix makes the case that thyroid hormone signaling bridges the environment and gene programs needed for brain development—and that environmental chemicals that disrupt normal thyroid function pose significant risks to the inherited intelligence and mental health of future generations. The first chapter provides an historical overview of documented cases in which environmental pollution has caused IQ loss across populations. The following chapters explain the physiology of thyroid hormone action, the importance of iodine and selenium for thyroid hormone signaling and brain development, and why thyroid hormone is such a sensitive target for environmental pollution. The final chapters discuss the role of gene-environment interactions in neurodevelopmental disorders and address what can and must be done by individuals, associations, and decision-makers to staunch these epidemics.