Mammary-type myofibroblastoma (MFB), also named mammary and extramammary myofibroblastoma, was first termed myofibrolastoma of the breast, or, more simply, either mammary myofibroblastoma (MMFB) or just myofibroblastoma. The change in this terminology occurred because the initial 1987 study and many subsequent studies found this tumor only in breast tissue. However, a 2001 study followed by numerous reports found tumors with the microscopic histopathology and other key features of mammary MFB in a wide range of organs and tissues. Further complicating the issue, early studies on MFB classified it as one of various types of spindle cell tumors that, except for MFB, were ill-defined. These other tumors, which have often been named interchangeably in different reports, are: myelofibroblastoma, benign spindle cell tumor, fibroma, spindle cell lipoma, myogenic stromal tumor, and solitary stromal tumor. Finally, studies suggest that spindle cell lipoma and cellular angiofibroma are variants of MFB. Here, the latter two tumors are tentatively classified as MFB variants but otherwise MFB is described as it is more strictly defined in most recent publications. The World Health Organization, 2020, classified mammary type myofibroblastoma tumors and myofibroblastoma tumors (i.e. extramammary myofibroblastic tumors) as separate tumor forms within the category of fibroblastic and myofibroblastic tumors. Mammary MFB likely represents less than 1% of all breast tumors. Extramammary MFB, however, has in recent studies been found to occur far more frequently than mammary MFB: a study of 143 patients reported that extramammary MFB outnumbered mammary MFB 10 to 1. Hence, the overall disease may be more common than previously considered. Extramammary MFB occurs about equally in males and females of both sexes and has a broad age distribution that includes children. Mammary MFB likewise occurs about equally in both sexes but has a decided predominance in middle-aged and older adults. MFB are completely benign tumors, i.e. they do not metastasize and when surgically removed rarely recur. Microscopically, they consist of neoplastic spindle cells, i.e. cells that are longer than wide, have a morphology somewhere between fibroblasts and myofibroblasts, have similarly appearing counterparts in normal tissues, and in normal tissues are usually identified as fibroblasts. The neoplastic cells commonly: 1) have acquired a gene chromosome abnormality in which a small part of chromosome 13 is deleted; 2) fail to express the retinoblastoma protein (pRb) due to this deletion; and 3) contain key tumor marker proteins. A tumor with this characteristic microscopic appearance, 13q14 deletion, loss of pRb,