<b><i>Background:</i></b> According to the latest WHO classification, neuroendocrine neoplasm (NEN) G3 of the gastrointestinal tract is defined by a proliferation index Ki67 above 20%. Gastrointestinal neuroendocrine carcinoma (NEC) is a rare and highly aggressive malignancy and despite responsiveness to chemotherapy, overall survival is poor. In the last 3-4 years, the heterogeneity of the NEN G3 group has become evident. <b><i>Summary:</i></b> In addition to the proliferative activity, the tumour differentiation seems to play a major role, further dividing the NEN G3 group into neuroendocrine tumour (NET) G3 and NEC. NET G3 often arise in the pancreas, and their median proliferation rate is lower and prognosis is better as compared to NEC. However, NET G3 show a limited response to platinum-based chemotherapy. Lack of specific data for NET G3 hampers clear therapeutic recommendations. Cisplatin combined with etoposide is the established standard regimen for advanced gastrointestinal NEC. Substituting carboplatin for cisplatin or irinotecan for etoposide is considered alternative first-line regimen. There is no standard second-line treatment; options are discussed within this review. <b><i>Key Points:</i></b> (1) In NEN G3, the distinction between NET G3 and NEC G3 is clinically and prognostically meaningful. (2) Platinum-based chemotherapy remains the recommended first-line treatment in metastasized NEC patients. (3) There is no established standard for NET G3; treatments established for NET G2 such as temozolomide-based chemotherapy or peptide receptor radiotherapy may be considered. (4) Specific trials for NET G3 are necessary. (5) New therapies for NEC are urgently needed. Checkpoint inhibitors should be evaluated.