This chapter presents a state of the art summary of thyroid tumors relation to its biology, epidemiology, disease mechanisms, clinical signs, diagnosis, treatment and prognosis. Thyroid carcinoma is the most common malignancy of the endocrine system, accounting for 1%–3% of all tumors worldwide. It has an annual incidence of 9–12 cases per 100,000 and shows a bias for women. Diagnosis of thyroid tumors involves medical history review, physical examination, laboratory investigation, imaging studies, and cytologic or histologic examination. Medullary thyroid carcinoma is a gray-tan-yellow, firm, solid, typically nonencapsulated, poorly differentiated tumor without a well-formed capsule. Follicular thyroid carcinoma is a gray-tan-pink, solitary, encapsulated, well-differentiated tumor with focal hemorrhage, variable fibrosis, and calcification. Papillary thyroid carcinoma is a solid, firm, gray-white, often multifocal, encapsulated, or infiltrative, well-differentiated tumor with central scar, cysts, fibrosis, and calcification. Tumors affecting the thyroid gland include epithelial tumors, nonepithelial tumors, and secondary tumors.