Neuroendocrine neoplasms of the gut are a heterogeneous group of tumors showing different morphological, clinical, prognostic, and molecular features. Currently, these proliferations are classified as “neuroendocrine neoplasms” (NENs), which include two main entities: well-differentiated neuroendocrine tumor (NET) and poorly differentiated neuroendocrine carcinoma (NEC). It is worth noting that a growing burden of evidence has been accumulating in the last years showing that NET and NEC, despite sharing neuroendocrine differentiation, are, in fact, two different diseases with different morphology, clinical presentation, outcome, and molecular background. NETs are graded with a three-tiered system (NET G1, NET G2, NET G3) based on the proliferation assessment, using both mitotic count and Ki67 proliferation index. NECs, which show a poorly differentiated morphology, are high grade by definition. In addition to pure NENs, mixed epithelial neoplasms composed of a neuroendocrine and a non-neuroendocrine component can be found in the digestive system. When each of the two components is malignant and reaches at least 30% of the tumor mass, these neoplasms are defined as mixed neuroendocrine–non-neuroendocrine neoplasms (MiNENs). There are different MiNEN types which show different morphology and prognosis mainly based on the clinicopathological characteristics of each component. While the global incidence of malignant neoplasms is more or less stable over the last 40 years, the incidence and prevalence of NENs has increased over the same period, probably reflecting the effectiveness of modern diagnostic tools with consequent frequent detection of early-stage cases. Due to the early detection and the use of more effective personalized therapeutic approaches, survival of patients with NETs has been improving over time.