Neuroendocrine tumors comprise a family of neoplasms with a wide range of morphologic, functional, and behavioral characteristics. Their diagnosis depends on the recognition of characteristic morphologic features and on the presence of markers indicative of neuroendocrine differentiation. Neuroendocrine tumors can be grouped into epithelial and neural subtypes on the basis of the presence of cytokeratins or neurofilaments, respectively. The nomenclature of these tumors, particularly those of the epithelial type, remains controversial. For some tumor types, such as those of the anterior pituitary, thyroid, and parathyroid glands, the standard terminology is preferable to the terms neuroendocrine tumor or neuroendocrine carcinoma. The use of the term carcinoid, however, has become increasingly more inappropriate to describe the full range of neuroendocrine neoplasms of low to intermediate malignant potential. It is critical to convey in diagnostic teminology both the tumor type and its malignant potential. Diagnostic terms using neuroendocrine or endocrine together with information on grading and hormonal activity provide an optimal approach for the classification of these tumors. Additional studies, including gene expression profiling, will be critical for further advancing our understanding of the pathobiology and interrelationships of these neoplasms.