Retroperitoneal lesions represent a broad, diverse collection of entities; when they contain fat, the differential diagnosis, which ranges from benign to fully malignant lesions, substantially narrows. Lipomas rarely occur in the retroperitoneum; thus, fat-containing lesions in this location should never be dismissed as lipoma. Pelvic lipomatosis is the overgrowth of histologically normal fat in the extraabdominal compartments of the pelvis along the perirectal and perivesicular spaces. Infants and young children develop lipoblastomas rather than liposarcomas, which occur in older patients. Liposarcomas typically occur in patients 50-70 years old and manifest in multiple subtypes, with the most common being well-differentiated liposarcoma. Liposarcomas are histologically and radiologically protean, with no one imaging feature specific across the spectrum of subtypes. Hibernoma is a rare benign soft-tissue tumor composed of brown fat. Because hibernomas contain varied portions of brown and white fat as well as lesser amounts of myxoid material and spindle cells, their imaging features vary considerably. Teratomas are neoplasms that originate in pluripotent cells--benign or malignant germ cells--that give rise to a wide spectrum of mature or immature tissues that are foreign to the location in which they arise and which demonstrate varying amounts of organ formation. Myelolipoma, a benign tumor composed of mature fat and interspersed hematopoietic elements that resemble bone marrow, typically originate in an otherwise normal adrenal gland. Angiomyolipoma is composed of varying admixtures of blood vessels, smooth muscle cells, and adipose tissue; any one or two of these elements may predominate. Although these fat-containing lesions have overlapping radiologic features, use of demographic and clinical data helps refine the diagnostic options and treatment.