Background: The goal of mastectomy is remove all of the glandular breast tissue (BGT) without compromising skin flaps viability. The purpose of this systematic review was to localize and/or estimate the amount of residual BGT (rBGT) after mastectomy and to identify factors that could be related to rBGT and/or residual disease. Methods: We conducted a PubMed search. The report was subdivided according to the method used to evaluate the presence of rBGT. A total of 16 publications were included in our final report. Two main methods for identifying rBGT included imaging (i.e., MRI scan) and histological evaluation at the time of mastectomy. Results: The rate of rBGT was reported in up to 100% of the patients and was found to be associated mainly with the type of surgical resection, indication, and surgeon’s expertise. Residual breast tissue can be found in all areas of the remaining chest wall, mostly in the skin-flaps, and more frequently underneath the nipple-areola complex.