Many of the major solid, malignant tumors of childhood have histologic similarities that reflect their dysembryonic and primitive features. One subset of these neoplasms, Ewing's sarcoma (ES) and primitive neuroectodermal tumor (PNET), presents primarily in the bone and soft tissues. Both tumor types were reported at a time and date well before the advent of electron microscopy and immunohistochemistry. Opposition to ES and PNET as distinctive entities developed and persisted because these tumors were considered incompletely documented examples of metastatic neuroblastoma or malignant lymphoma. General acceptance of ES as a unique tumor type occurred well before the PNET had been fully defined and characterized. Once these neoplasms had joined the other round cell neoplasms, the quest for the histogenesis was pursued, but the results were frustratingly inconclusive, especially for ES. Because of the resemblance of the PNET to classic neuroblastoma, the neural crest was regarded as the most likely progenitor. With the recognition of osseous PNET, extraosseous ES, and a shared cytogenetic abnormality between ES and PNET, more recent speculation has focused on the possibility that these presumably separate neoplasms are closely related histogenetically without directly answering the question of histogenesis. Despite the likely common progenitorship of ES and PNET, the latter neoplasm is seemingly the more aggressive. Although melanotic neuroectodermal tumor of infancy, intra-abdominal desmoplastic small cell tumor, and polyphenotypic small cell tumors have some overlapping microscopic and immunohistochemical features with PNET, their relationship to ES-PNET has otherwise not been resolved.