In the preoperative assessment of thyroid nodules, ultrasonography and ultrasonography-guided fine needle aspiration biopsy play the most important role, especially for papillary thyroid cancer. The problem to differentiate follicular adenoma from highly differentiated follicular carcinoma remains the problem in preoperative diagnostic. Also the additional use of a multi tracer imaging strategy (Tl-201/Tc-99 m subtraction scan, Tc-99 m Sestamibi, Tc-99 m Tetrofosmin dual phase scintigraphy) has not solved this problem. Although it is unlikely, the question whether F-18-fluorodeoxy-glucose-positron emission tomography is able to give a better differentiation between benign and malignant tumors in the preoperative assessment of thyroid nodules is not answered up to now. In contrast to preoperative diagnostics F-18-fluorodeoxyglucose-positron emission tomography is of great value in the postoperative follow up of differentiated thyroid cancer. In case of elevated serum thyroglobulin but negative I-131 WBS F-18-fluorodeoxy-glucose-positron emission tomography is the method of choice to detect I-131 negative recurrences and metastases. F-18-fluorodeoxy-glucose uptake in metastases from differentiated thyroid cancer is correlated to low differentiation and maybe bad prognosis. There is also evidence that F-18-fluorodeoxyglucose-positron emission tomography may have a role of in anaplastic and especially in medullary thyroid cancer in the future.