Sinonasal tract neoplasms composed of light microscopically seemingly "undifferentiated" small round cells often generate considerable diagnostic difficulty. Although the careful review of H&E-stained sections remains of critical and central importance in this evaluation, the recent improvements in the immunohistochemical diagnostic armamentarium and molecular diagnostic techniques applicable to paraffin-embedded tissue samples may add diagnostically valuable information. Accordingly, this review will discuss the differential diagnosis of undifferentiated small blue cell tumors of the sinonasal tract based on the light microscopic and clinical features and, as needed, the results of these ancillary studies. Tumors discussed include olfactory neuroblastoma, sinonasal undifferentiated carcinoma, small cell undifferentiated (neuroendocrine) carcinoma, undifferentiated (lymphoepithelioma-like) carcinoma, malignant melanoma, pituitary adenoma, Ewing sarcoma/peripheral neuroectodermal tumor, rhabdomyosarcoma, mesenchymal chondrosarcoma, small cell osteosarcoma, synovial sarcoma, extranodal natural killer/T-cell lymphoma, nasal type, and extramedullary plasmacytoma.