One of the main reasons for publishing this second edition of “T- roid Cancer” is the fact that the first edition has sold out. Furth- more, during the 4 years following the publication of the first edition in 2001, some progress, mainly in the basic sciences (molecular biology), has been made. The most prominent change in the clinical sciences has been the new classification of thyroid cancer, especially with relation to T1–T3 tumors. Now, tumors with a diameter of up to 2 cm are still classified T1. This new UICC classification (6th edition) follows the classification of the American Society of Pathology. These changes require a modification of the old guidelines. According to the Hedinger classification (1988) tumors with a diameter below 1 cm were classified as “papillary microcarcinoma of the thyroid”. Only in those tumors was total or nearly total thyroidectomy deemed unn- essary and I-131 therapy not a prerequisite for treatment. The majority of the chapters has been updated including references to many new publications. Two new chapters, on I-124 PET and - simetry, have been added. We strongly feel that this second edition of “Thyroid Cancer” is again a state-of-the-art overview of the diagnosis and treatment of thyroid cancer. Bonn, Frankfurt am Main H. -J. Biersack, F. Grünwald Preface to the First Edition Thyroid cancer was first described at the end of the eighteenth c- tury.