Tumors arising from neuroendocrine cells are defined as epithelial neoplasms with predominantly neuroendocrine differentiation. They comprise a distinct group of tumors with a characteristic histological structure and functional properties that develop at various sites, particularly the gastrointestinal system (67%) and lungs (25%). Although such tumors are usually slow-growing and indolent, almost all have malignant potential and most can produce active hormones. Clinical signs vary, and many are dependent on the site at which the tumor develops. Although these tumors were identified more than 130 years, their classification remains unclear.This review provides a comprehensive overview of the human neuroendocrine system and its neoplasms, from their discovery to current terminology and classifications. In addition, the clinical symptomatology and macroscopic/microscopic features of tumors arising from endocrine cells of the gastrointestinal tract are described, with an emphasis on their classification, diagnostic criteria for their grading and TNM (tumor, node, metastasis) staging, and how these tumors differ according to their localization in the gastrointestinal tract.Tumors arising from neuroendocrine cells are rare and can cause typical symptoms of carcinoid syndrome. However, most of these tumors are asymptomatic, which, together with their typical small size and localization in the gut, makes them difficult to access endoscopically and often leads to diagnosis at an advanced stage. To successfully diagnose and treat tumors arising from neuroendocrine cells, they should be assessed using a differential diagnostic procedure and be histopathologically classified, graded, and staged according to specified criteria and the latest classifications and guidelines. Although the terms "carcinoid", "neuroendocrine tumor", and "neuroendocrine carcinoma" are often used synonymously in the literature and by professionals, more precise terminology is required for nomenclature and classification. Key words: gastrointestinal neuroendocrine neoplasms - neuroendocrine tumors - neuroendocrine carcinomas - classification - NET - NEC The authors declare they have no potential conflicts of interest concerning drugs, products, or services used in the study. The Editorial Board declares that the manuscript met the ICMJE recommendation for biomedical papers. Submitted: 21. 3. 2018 Accepted: 16. 4. 2018.