Known aetiological factors of thyroid tumours are: genetic, hormonal, irradiation and auto-immune thyroiditis. Some medullary carcinomas are familial. Thyroid-stimulating hormone plays a significant part in the initiation and maintenance of many well-differentiated thyroid carcinomas. Small doses of irradiation in childhood and probably also in adolescence may induce a papillary or follicular carcinoma. Some malignant lymphomas develop in a previous auto-immune thyroiditis. Diagnosis of a malignant thyroid tumour is usually obvious once there has been extrathyroidal spread. Intrathyroidal tumours are suspect if they are hard and irregular. The real diagnostic problem is the clinically solitary nodule which is smooth, firm and mobile in a euthyroid patient. Ultrasound and scanning may increase or decrease the suspicion of malignancy but excisional biopsy is the only certain diagnostic procedure.