Microglandular adenosis is widely known as a benign breast lesion that can produce a mass. It differs from other types of adenosis by having an infiltrative pattern of growth and glands lacking a myoepithelial layer. Although in every other aspect the cells lining the glands are epithelial, they stain strongly with S100 protein. Most cases of microglandular adenosis are resolved after adequate excision, but more recent data suggest that this lesion may be a precursor for carcinoma, with atypical microglandular adenosis being an intermediate lesion. Carcinomas arising in a background of microglandular adenosis, although they may be low grade, are commonly estrogen and progesterone receptor negative, in contrast to most conventional low-grade carcinomas unrelated to microglandular adenosis. They also show immunopositivity for S100 protein, similar to microglandular adenosis. Diagnostic problems include differentiating microglandular adenosis from carcinoma and assessing the extent of the carcinoma component. In this review, we discuss the histomorphologic and immunophenotypic characteristic of the spectrum of microglandular adenosis lesions with emphasis on diagnostic features distinguishing microglandular adenosis from well-differentiated carcinoma, in particular, tubular carcinoma, and assessment of surgical margins of excision in such lesions.