Differentiated thyroid carcinoma originates from thyroid follicular cells and is the most prominent malignancy of the endocrine organs. There are two histological types of differentiated carcinoma, namely, papillary and follicular carcinoma. According to reports from Western countries, papillary carcinoma comprises 85.3% of thyroid malignancies in whites, and 72.3% in blacks. In Japan, a previous study showed that the prevalence of papillary carcinoma was 78.4% based on material registered between 1977 and 1986, but according to recent findings reported in 2004 by Japanese Society of Thyroid Surgeons (JSTS), papillary carcinoma accounted for as much as 93% of all thyroid carcinomas. Papillary carcinoma frequently metastasizes to the regional lymph node and shows multicentricity in the thyroid gland. It usually shows a typical ultrasonographic appearance and can be rather easily diagnosed by fine needle aspiration biopsy (FNAB). Follicular carcinoma accounts for 10.9-20.5% of the patients in the United States. In Japan, the prevalence of follicular carcinoma was reported to be 17.2%, but it decreased to 5% in a report by JSTS in 2004. This carcinoma is only occasionally diagnosed preoperatively, because it is hard to discriminate follicular carcinoma from benign adenoma on imaging studies and cytologic findings. In contrast to papillary carcinoma, follicular carcinoma more often metastasizes to distant organs than regional lymph nodes. In Japan, the prevalence of papillary carcinoma increased and that of follicular carcinoma decreased between reports from 1977 to 1986 and that in 2004, which may be because follicular variant of papillary carcinoma was classified into follicular carcinoma in the previous results. Generally, these carcinomas show an indolent character, but when the lesion dedifferentiates and becomes undifferentiated carcinoma, it displays very rapid growth with an adverse prognosis and is regarded even as the most aggressive malignancy among human solid carcinomas. Furthermore, cases showing certain characteristics are likely to be constantly progressive and even become life-threatening. Such cases should be regarded as "high-risk" requiring careful and extensive surgical treatment and postoperative follow-up. Indeed, it is most important for physicians to correctly distinguish high-risk cases from those with an indolent character, although how to evaluate the biological characteristics of thyroid carcinoma and how to identify high-risk cases remains highly controversial. In this review, the methods of distinguishing high-risk cases and the appropriate therapeutic strategies for papillary and follicular carcinomas predominantly based on our experience are emphasized and our proposals for therapies including surgical treatment are demonstrated.