The annual incidence of neuroendocrine tumours in the Caucasian population ranges from 2.5 to 5 new cases per 100,000 inhabitants. Gastroenteropancreatic neuroendocrine tumours vary considerably in their hormonal composition, the syndromes they cause and their biological behaviour. This high complexity and clinical heterogeneity, together with the well-known difficulty of predicting their behaviour from their pathological features, are reflected in the many classifications that have been formulated over the years. This article reviews the main tissue and clinical biomarkers and makes recommendations for their use in medical practice. This document represents a consensus reached jointly by the Spanish Society of Medical Oncology (SEOM) and the Spanish Society of Pathology (SEAP).