Microinvasive breast carcinoma is a rare lesion, in which foci of intralobular or intraductal carcinoma are associated with one or more microscopic foci of atypical cells located outside the basement membrane, in the adjacent intralobular or interlobular stroma . In rare cases, microinvasive carcinoma can be identified without an adjacent in situ lesion (Fig. 12.1). Frequently, microinvasive carcinomas have a multifocal character. There is no international consensus regarding this lesion and its definition and all definitions are arbitrary. Some authors consider that the maximum size of the invasive focus should be 1 mm for the diagnosis of microinvasive carcinoma . Other authors define microinvasive carcinoma as having a single focus with a maximum size less than 2 mm, while others consider 2–3 foci, none exceeding 1 mm in diameter . However, the presence of microinvasive breast carcinoma and distinction from in situ carcinoma may have therapeutic and prognostic implications, consequently recognizing it is of paramount importance. During microscopic examination, the pathologist establishes microinvasion on the basis of several morphological criteria. Sometimes, these criteria may be difficult to assess and may not be perfectly reproducible among pathologists.