Poorly differentiated (PD) carcinomas of the thyroid represent an heterogeneous but distinct group of tumors, clinically and histopathogenetically intermediate between follicular-derived well-differentiated and anaplastic carcinomas. Although the diagnostic criteria for inclusion in the PD tumor group are far from well established, and despite controversies on nomenclature, the identification of PD carcinomas as tumors with trabecular/insular/solid (TIS) growth patterns and high-grade histopathological parameters is generally accepted. Recent data on large tumor series were focused on the recognition of clinicopathological features able to predict aggressive behavior. Patient age >45 yr, the presence of necrosis (either focal or extensive), and mitotic count >3 per 10 HPF have been found to be the most influent prognostic parameters. Therefore, as also proposed for other thyroid carcinomas (e.g., papillary carcinoma) grading of PD carcinomas on the basis of these latter parameters is encouraged to select those cases with a high risk of poor outcome.