Here, we review the role of clinical biomarkers (tissue and circulating markers) in the management of neuroendocrine tumors. These tumors may originate in different organs, from cells embriologically different but expressing common phenotypic characteristics, such as the immuno-reactivity for markers of neuro endocrine differentiation (defined as "pan-neuroendocrine"), the capacity to sec rete specific or aspecific peptide and hormones, and the expression of some receptors, that are at the basis of the current diagnostic and therapeutic approach.