The majority of fat-containing masses are asymptomatic, but can present as painless, soft, mobile masses. A fat-containing mass within the breast is a benign finding, with the exception of the rare liposarcoma. The differential in non-lactating woman includes a lipoma, hamartoma, lymph node, and fat necrosis. Any atypical presentation of a fat-containing mass (such as thick, nodular septation or capsule, large size >10 cm, a mass that is clinically increasing in size) should raise suspicion for a rare fat-containing tumor such as an atypical lipomatous tumor or well-differentiated liposarcoma. This chapter reviews the key clinical and imaging features, imaging protocols and pitfalls, differential diagnoses, and management recommendations for a fat-containing, circumscribed mass. Topics discussed include lipomas, hamartomas, oil cysts, fat necrosis, and steatocystoma multiplex. Fat-containing lesions in the setting of lactation (galactocoele) as well as intramammary lymph nodes are discussed in separate chapters.