Gastroenteropancreatic (GEP) neuroendocrine neoplasms are classified as low-grade, intermediate-grade, and high-grade tumors based on morphologic criteria and the proliferation rate. Most studies have been conducted in patients with well differentiated (low-grade to intermediate-grade) neuroendocrine tumors. Data are substantially scarcer on poorly differentiated, high-grade neuroendocrine carcinoma (NEC), which includes the entities of small cell carcinoma and large cell NEC. A literature search of GEP-NEC was performed. Long-term survival was poor even among patients who presented with localized disease. Several studies highlighted heterogeneity within the high-grade NEC category and a need for the further identification of discreet prognostic and predictive groups. Tumors with a Ki-67 proliferation index <55% were less responsive to platinum-based chemotherapy, and patients with such tumors or with well differentiated morphology had better survival than patients who had tumors with poorly differentiated morphology or a higher Ki-67 index. Treatment options beyond platinum-based chemotherapy are emerging. A revision of the World Health Organization high-grade NEC classification seems to be necessary based on recent data. Platinum-based chemotherapy may not be the optimal treatment for patients who have GEP-NEC with a moderately high proliferation rate. Adequate diagnostic and prognostic stratifications constitute the basis for future progress. Cancer 2014;120:2814–2823. © 2014 American Cancer Society.