A great majority of adrenocortical tumors are benign, and many adrenocortical carcinomas (ACC) are obviously malignant at presentation. The histopathological diagnosis of ACC is occasionally difficult, particularly with stage I and stage II disease. The prognosis of ACC is generally poor. Surgery is the major treatment, with chemotherapy and radiotherapy being applicable to only restricted patients. The Weiss criteria are useful in diagnosing the common adult type of ACC. Histopathological prognostic factors of ACC have not been fully established because of the rarity of the disease. In this article, we first describe the current histopathological diagnostic and prognostic factors of ACC, highlighting the special types of ACC to which Weiss's criteria are not fully applicable. These special type tumors include pediatric adrenocortical tumors, oncocytomas, and aldosterone-producing tumors of pure zona glomerulosa type. Then we present three cases with unusual small adrenocortical tumors. One patient had an unequivocal ACC showing metastatic disease. One had a histologically defined ACC with no metastasis or macroscopic invasion. The third was a pediatric patient with a tumor showing a nodule-in-nodule pattern with insulin-like growth factor II expression.