Poorly differentiated thyroid carcinomas are a heterogeneous group of tumors occupying an area intermediate between well-differentiated follicular or papillary carcinoma and anaplastic carcinomas, from both a histopathogenetic and a clinical point of view. Large tumor series selected on the basis of structural and/or other morphological criteria showed that poorly differentiated carcinomas have a distinct biological behavior, and the classification of these tumors into a separate group appears justified, although strict homogeneous diagnostic criteria should be achieved and widely accepted to better characterize such tumor entity. Moreover, the identification of the prognostic parameters segregating aggressive from indolent cases has important clinical implications. Molecular data in the literature, although limited by the heterogeneous case series analyzed, identify ras alterations as the most common molecular alteration in poorly differentiated carcinomas, thus, depicting a peculiar molecular pathway in this tumor type as compared to well-differentiated follicular and papillary carcinomas. The present paper aims to review the various aspects of this tumor type, from morphology to immunohistochemistry and molecular abnormalities from a practical and daily practice-oriented point of view.