Borderline malignancy has become a popular but very loosely used term. It is used in a variety of soft tissue tumors to connote different biologic behaviors or potential and, as such, it has ceased to be a meaningful concept, even if it may be useful as an expression of uncertainty in daily practice. In reviewing soft tissue tumors that are commonly regarded as borderline malignant, it seems that much uncertainty seems from failure to properly define malignancy and to understand mechanisms of malignant behavior in the specfic setting of mesenchymal neoplasia.