The autoimmune thyroid diseases and familial thyroid cancers are the current target of molecular thyroid genetics. Unlike the situation in monogenic thyroid diseases, for which the molecular genetics has largely been clarified over the past 20 years, a methodological approach to these more complex forms of thyroid disease has not yet been well established. The determination of susceptibility genes, for example, remains a major challenge. The contributors to this volume are attempting to meet that challenge in research on molecular genetics. Meeting at the first International Symposium on the Genetics of Complex Thyroid Diseases, held in Kyoto, Japan, 20 distinguished researchers from five countries in addition to Japan shared their latest results and engaged in intense discussion, focusing on the autoimmune thyroid diseases and familial thyroid cancers. Their papers collected here are a valuable contribution to the field of the genetics of complex thyroid diseases.