The WHO definition of the 3 classes of well differentiated endocrine tumor, well differentiated endocrine carcinoma and poorly differentiated carcinoma, allows an efficient general classification of GEP NETs fitting European Union (EU) current clinical use. The ENETS grading and TNM staging systems are complementary tools allowing to improve patients' stratification, and their adoption is strongly recommended. However, the prognostic value of these tools is still under investigation and more data are needed to support their usefulness at all gut sites and in perspective series. Similarly, the four classes system adopted by the WHO for the endocrine tumors of the lung allows a prognostically effective stratification of patients. A common grading system is advocated for lung and GEP NETS.