Primitive round cell neoplasms (small round cell tumors) of soft tissue are a diverse group of malignant tumors composed of monotonous undifferentiated cells with high nuclear-cytoplasmic ratio. Many occur more frequently, although not exclusively, in childhood. As tumors with primitive round cell morphology are seen in virtually every basic tumor category, the diagnosis of small round cell neoplasms requires the use of ancillary diagnostic techniques: immunohistochemistry and often molecular genetics. The principal tumors in this group include Ewing sarcoma/primitive neuroectodermal tumor, desmoplastic small round cell tumor, alveolar rhabdomyosarcoma, poorly differentiated synovial sarcoma, neuroblastoma, and ganglioneuroblastoma.