Medullary carcinoma of the thyroid (MCT), the tumor entity related to parafollicular C cells of the thyroid gland, is characterized by peculiar morphological, functional and behavioral features. Apart from the constant finding of calcitonin production by MCT, heterogeneous patterns of growth as well as different associations with other lesions in the setting of MEN II syndrome have been observed. This review addresses these points, as were discussed at the 1992 European School of Pathology courses. Heterogeneity of histological patterns of MCT suggests caution in the interpretation of any thyroid tumor. Immunohistochemical demonstration of calcitonin is apparently the only valid criterion for a differential diagnosis. A correct identification of MCT is important for the individual patient (to plan proper therapy) but also for patient's relatives to screen other familiar cases in the setting of MEN II syndromes.