Intraperitoneal solid tumors are far less common in children than in adults, and the histologic spectrum of neoplasms of the peritoneum and its specialized folds in young patients differs from that in older patients. Localized masses may be caused by inflammatory myofibroblastic tumor, Castleman disease, mesenteric fibromatosis, or other mesenchymal masses. Inflammatory myofibroblastic tumor is a mesenchymal tumor of borderline biologic potential that appears as a solitary circumscribed mass, possibly with central calcification. Castleman disease is an idiopathic lymphoproliferative disorder that appears as a circumscribed, intensely enhancing mass in the mesentery. Mesenteric fibromatosis, or intra-abdominal desmoid tumor, is a benign tumor of mesenchymal origin associated with familial adenomatous polyposis. Mesenteric fibromatosis appears as a mildly enhancing, circumscribed solitary mass without metastases. Diffuse peritoneal disease may be due to desmoplastic small round cell tumor (DSRCT), non-Hodgkin lymphoma, or rhabdomyosarcoma. DSRCT is a rare member of the small round blue cell tumor family that causes diffuse peritoneal masses without a visible primary tumor. A dominant mass is typically found in the retrovesical space. Burkitt lymphoma is a pediatric tumor that manifests with extensive disease because of its short doubling time. The bowel and adjacent mesentery are commonly involved. Rhabdomyosarcoma may arise as a primary tumor of the omentum or may spread from a primary tumor in the bladder, prostate, or scrotum. Knowledge of this spectrum of disease allows the radiologist to provide an appropriate differential diagnosis and suggest proper patient management.