Ultrasonography is the most important imaging tool in the diagnosis of thyroid disease. The results of real-time B-imaging of the thyroid gland along with physical signs and basal TSH can aid in the diagnosis of thyroid dysfunction, of for instance, a small, hypoechogenic gland in Hashimoto's and radiation thyroiditis, or an enlarged, hypoechogenic and pulsating gland in Graves' disease. Although recent improvements in technology have increased sensitivity of colored duplexsonography, certain sonographic differentiation of benign and malignant lesions as well as of active (hormone secreting) and inactive nodules is not yet possible. Diagnostic interpretation of ultrasonographic findings is feasible only when the history of the patient, physical examination and the laboratory evaluation are taken into account. Ultrasound detects thyroid nodules, is useful for following nodule size, in guiding fine needle biopsies and in the aspiration of cysts. Highly experienced investigators in ultrasound can assist preoperatively in the localization of parathyroid adenomas in primary and tertiary hyperthyroidism (when followed by (99m)Tc szintigraphy plus SPECT).