This volume contains the proceedings of a conference held at the National Institutes of Health in Bethesda on March 21-23. 1988. jointly sponsored by the International Council for Control of Iodine Deficiency Disorders (ICCIDD) and the Fogarty International Center of the National Institutes of Health. Several themes converged to make this meeting timely. The first is an increasing awareness of iodine deficiency disorders as a world-wide problem of public health and a preventable cause of mental deficiency. and as a subject of scientific effort. Increased interest in these problems owes a great deal to accessibility to remote and under developed areas of the world where iodine deficiency persists. As with any subject. greater scrutiny yields unexpected complexity and interest. It is true that provision of iodine. typically as iodized salt, is the necessary and sufficient preventative for iodine deficiency disorders. without including endemic cretinism. This provision is a governmental, economic and social problem. Apart from this, however, the scientific and medical problem of iodine deficiency and its effect on brain development and function is one of great interest and importance for developmental neurology and psychology. Even though the specific preventative agent is known, we do not totally understand the neurobiological questions raised.