The increased rate of early detection of breast cancer due to widespread mammographic screening has led to an increased incidence not only of in situ but also microinvasive carcinoma (MC). MC has been reported to have a favourable prognosis, but specific definitions have varied in the past making the clinical significance of this entity a subject of debate. In fact, although the diagnosis of MC often appears in pathology reports, this term has not been used in a consistent, standardized manner. In addition, the histological diagnosis of MC can be problematical for the pathologist due to a variety of in situ patterns and artefacts that may be misinterpreted as stromal invasion. Definitions and diagnostic criteria of MC are reviewed and discussed. Based on a review of literature, incidence of axillary lymph node involvement, according to different definitions of microinvasion, is reported.