Deciding whether in-situ breast carcinoma is associated with microinvasion is a common problem. Histological features resembling invasion can be simulated by in-situ carcinoma distorted by inflammatory and reparative changes. Having expended the effort to diagnose genuine microinvasion, just how useful is this diagnosis in planning further treatment and follow-up? In the following articles, Hoda et al. comment on the utility of immunohistochemistry in resolving uncertainty about the presence of microinvasion, and Ellis et al. critically appraise the definition of microinvasion and its clinical significance.