Gastroenteropancreatic (GEP) neuroendocrine neoplasms (NEN) are classified based upon morphology and graded upon proliferation rate as either well-differentiated low-grade (G1-G2) neuroendocrine tumours (NET) or poorly differentiated high-grade (G3) neuroendocrine carcinoma (NEC). Recently, a new subgroup of well-differentiated high-grade pancreatic tumours (NET G3) has been defined. The GEP NEN G3 group consisting of both NEC and NET G3, has recently been shown to be a quite heterogeneous patient group concerning prognosis and treatment benefit, depending on factors such as primary tumour site, differentiation, proliferation rate and molecular alterations. In this review we discuss the existing data on diagnostics, treatment and biomarkers on this patient group, the unmet needs and the future perspectives.