Breast lymphoma is a rare hematologic neoplasm that originates in the breast lymphoid tissue and includes primary breast lymphoma (PBL) and secondary breast lymphoma (SBL). PBL involves the breast lymphoid tissue in the absence of previously identified extramammary lymphoma and widespread disease. SBL is the most common metastasis to the breast, accounting for 17% of metastatic disease to the breast. PBL and SBL usually demonstrate imaging phenotypes that overlap with those of primary breast carcinoma, which makes a prospective diagnosis of breast lymphoma challenging. These nonspecific imaging features include an iso- to hyperdense oval mass or masses at mammography, a hypoechoic or mixed-echogenicity hypervascular mass at US, an enhancing mass with type II kinetics at MRI, and high fluorine 18-fluorodeoxyglucose avidity at PET. In cases of suspected lymphoma, reviewing the clinical history, using appropriate biopsy techniques, and evaluating for multiplicity, bilaterality, and distant disease are critical for diagnosis and management. A patient with PBL generally has an earlier clinical presentation with a palpable abnormality and a solitary imaging finding. In contrast, multiple masses in an older patient and an occult clinical presentation favor an SBL diagnosis. ©RSNA, 2019.