The neuroendocrine epithelial neoplasms (NENs) include well-differentiated neuroendocrine tumors (WDNETs) and poorly differentiated neuroendocrine carcinomas (PDNECs). Whereas PDNECs are highly lethal, with localized Merkel cell carcinoma somewhat of an exception, WDNETs exhibit a range of "indolent" biologic potentials-from benign to widely metastatic and eventually fatal. Within each of these 2 groups there is substantial morphologic overlap. In the metastatic setting, the site of origin of a WDNET has significant prognostic and therapeutic implications. In the skin, Merkel cell carcinoma must be distinguished from spread of a visceral PDNEC. This review intends to prove the thesis that determining the site of origin of a NEN is clinically vital and that diagnostic immunohistochemistry is well suited to the task. It will begin by reviewing current World Health Organization terminology for the NENs, as well as an embryologic and histologic pattern-based classification. It will present population-based data on the relative frequency and biology of WDNETs arising at various anatomic sites, including the frequency of metastases of unknown primary, and comment on limitations of contemporary imaging techniques, as a means of defining the scope of the problem. It will go on to discuss the therapeutic significance of site of origin. The heart of this review is a synthesis of data compiled from >100 manuscripts on the expression of individual markers in WDNETs and PDNECs, as regards site of origin. These include proteins that are considered "key markers" and others that are either useful "secondary markers," potentially very useful markers that need to be further vetted, or ones that are widely applied despite a lack of efficacy. It will conclude with my approach to the metastatic NEN of unknown origin.