Neuroendocrine tumors (NETs) were previously often called carcinoids and were divided into foregut, midgut, and hindgut carcinoids [1]. This classification did not relate to the natural history and clinical behavior of these tumors, so the classification has been revised several times taking into account tumor location, tumor size, angioinvasion, hormone production, histological grade, and proliferative index [2]. Using the present World Health Organization (WHO) classification of 2010, neuroendocrine neoplasms of the gastrointestinal tract are classified into grade 1 (G1), 2 (G2), or 3 (G3) tumors based on Ki67 index and mitotic count. G1 and G2 tumors are designated NETs, and G3 tumors neuroendocrine carcinoma (NEC). G3 tumors have a more aggressive clinical behavior, but G1 and G2 tumors can also give rise to metastasizing disease. NETs are often diagnosed at a late stage, when the patients have disseminated disease and hormonal symptoms. The workup includes computed tomography (CT) or MRT, somatostatin receptor (SSTR) imaging (111indium-octreotide scintigraphy or positron emission tomography [PET] using 68gallium-DOTA-conjugated peptides), hormonal screening, and analyses of tumor biopsies. The histopathological examination aims to classify the tumor with respect to origin and biological behavior. Immunohistochemical staining for amine and peptide hormones and synaptic vesicle proteins can be helpful in defining the origin of the tumor [3]. Analysis of growth pattern, degree of neuroendocrine differentiation, and proliferative capacity provides prognostic information [4] and aids in the selection of treatment. The neuroendocrine differentiation is evaluated by antibodies against proteins of the neurosecretory granules (e.g., chromogranin A and synaptophysin) or cytosolic markers (neuron-specific enolase or PGP 9.5). NETs are immunopositive for all markers in the majority of tumor cells, while NECs often lack the granular markers with cytosolic markers retained. Other factors, e.g., angiogenic capacity, expression of adhesion molecules, and specific genetic changes, are prognostic tools and indicators of responsiveness to medical treatment.