Knowledge of tumours of the thyroid gland has advanced consider- ably in the 22 years that have elapsed since work was started on the first edition of Histological Typing of Thyroid Tumours. In the intro- duction to that volume it was recognized that the definitions and clas- sifications put forward would need revision in time, and the present text differs substantially from the first edition. As far as is possible, however, the framework of the classification proposed remains the same, as the original classification was widely accepted and proved useful in many studies. The link between the morphological type of thyroid tumour and its epidemiology, natural history, function, prognosis and response to therapy has been further strengthened since the first edition. In par- ticular, the decision taken to separate papillary and follicular carcino- mas and exclude a mixed papillary follicular type has been well justi- fied. One of the major changes has been the recognition that many tu- mours regarded 20 years ago as small cell carcinoma were really ma- lignant lymphoma, and this development has been incorporated into this edition, with increased importance given to primary malignant lymphoma of the thyroid. Much work has also been done on medul- lary carcinoma of the thyroid, its link with mUltiple endocrine neo- plasia syndromes, and its association in its inherited form with C-cell hyperplasia; this too is recognized by an expanded section on this tu- mour.