The most frequent forms of thyroid cancer can be divided into two main categories: tumors with follicle cell differentiation and those with C-cell differentiation. Within the category of tumors with follicle cell differentiation, a distinction is made between papillary tumors and follicular tumors. Both undifferentiated and anaplastic tumors are also included in this category, although a clear attribution to an initial cell line is not always possible for anaplastic carcinomas. Within the largest group, the papillary carcinomas, there are several types including encapsulated, minimal invasive, diffuse sclerotic and oncocytic carcinomas. Because of their generally low iodine uptake and their high mito-chondrial content, Hürthle cell carcinomas have a special position, particularly in functional imaging and with respect to therapy options. Carcinomas with C-cell differentiation are partly genetically determined, either as isolated familial medullary carcinoma or in multiple endocrine neoplasia (MEN-2a/MEN-2b). The existence of mixed follicle cell/C-cell differentiated carcinomas should be mentioned, especially in the context of radioiodine therapy, since the latter can certainly offer therapeutic options. We will not discuss the rare forms of thyroid cancer or the further subcategoriza- tion of the tumor entities described above, since this information is currently not relevant to the clinical significance of PET.