Most neonatal abdominal masses will be due to benign retroperitoneal lesions such as hydronephrosis and multicystic dysplastic kidney. Although history and physical examination, plain radiographs and ultrasonography will confirm most diagnoses, severe unilateral hydronephrosis, hemorrhagic neuroblastoma, and intraperitoneal cysts may provide diagnostic difficulties. Masses identified by prenatal ultrasound need careful evaluation as they may represent normal structures, nonsignificant variants, or physiologically significant anomalies. Many lesions will require operative intervention, which can be safely performed in small infants by trained personnel at facilities with appropriate support services. Genuine controversy exists in the management of some of these lesions including MDK, renal vein thrombosis, and acalculous cholecystitis.