Atypical fibroxanthoma (AFX) or undifferentiated pleomorphic sarcoma (UPS) is a rare malignant neoplastic disease of the skin. At the beginning of the 1960s AFX was described as an independent entity and superficial variant of malignant fibrous histiocytoma (MFH). Since then, many controversies on the classification have arisen mainly because in many cases dedifferentiated neoplasms from other origins were falsely diagnosed as AFX. A relevant deep expansion, the invasion of nerves and vessels or the presence of tumor necrosis are described as being typical for UPS; however, in the first-line they represent risk factors for recurrence. In view of the clinical and histological features it is meaningful to consider AFX and UPS as one disease. In recent years many studies on the molecular pathological background have attempted to make a better classification of the neoplasm, without being able to so far name a certain specific histopathological or molecular pathological characteristic. The AFX/UPS is still in essence a morphological and immunohistochemical diagnosis by exclusion.