The cyclic structural changes leading to diffuse and nodular goiters are reviewed. Transition from benign to malignant thyroid tumor in man is a possibility but probably occurs rarely; papillary carcinoma is the most common differentiated thyroid tumor, papillary adenoma is the least common benign tumor, and both occur at about the same age. The differentiated group includes papillary, follicular and mixed tumors, and the undifferentiated group highly malignant small-cell and giant-cell carcinomas. The vascularity of the thyroid favors metastatic involvement. Therapy will be considered in part 3.