Neuroendocrine differentiation in breast neoplasms has been a matter of discussion since the first description of a neuroendocrine neoplasm (NEN) in this site. In fact, NENs of the breast (Br-NENs) represent a less well-defined group of neoplasms than analogous entities in other organs, such as the lung and the gastroenteropancreatic tract. Pure neuroendocrine phenotype is extremely rare, both for the well- and for the poorly differentiated morphology. In contrast, the expression of neuroendocrine markers in otherwise typical breast carcinomas, both of special and of non-special type, without morphologically evident neuroendocrine differentiation is more common. Consequently, the diagnostic criteria and the classification scheme for Br-NENs have been continuously changing over time, and real consensus on this topic is still lacking. In this chapter, we recapitulate the evolution of the concept of Br-NEN; review the available knowledge on their morphological, molecular, and clinical features; and critically discuss the current classification scheme.