The AA review the so-called prognostic factors in thyroid carcinoma taking in consideration four of the five major histotypes of thyroid tumours: Papillary ca. (5-year-survival -91%), follicular ca. (73%), poorly differentiated ca. (48%) and undifferentiated ca. (0%), with an emphasis on papillary carcinoma. The prognostic factors are described in two major groups: Clinicopathologic factors comprising the classic clinicopathologic features--age, gender, size, extrathyroidal extension, vascular invasion, nodal metastization and distant metastization--and Biologic factors, comprising a group of heterogeneous and less classic features such as DNA content and proliferation markers, oncogenes and tumour-suppressor genes, growth factors and growth factor receptors, pro- and anti-apoptotic genes, mucin and histo-blood group antigens, cytoskeletal filaments, adhesion molecules, and so forth. Finally, the AA address briefly a third group of (prognostic) factors that reflect the interaction between the neoplastic cells and their microenvironment: extracellular matrix and proteolytic enzymes (laminin, collagen IV, fibronectin, collagenases, alpha-1 anti-trypsin) and lymphocytic infiltration (S-100 positive cells, IGF-I and IGF-I receptor).