Although metastatic tumor to the breast is rare, with published reports between 0.2% and 3%, the diagnoses must be considered because patients with metastatic disease to the breast do not usually benefit from mastectomy, and delays in diagnosis contribute to the poor prognosis associated with most metastases to the breast from an extramammary location. Clinical history and presentation is of paramount importance and in many cases may be the only clue in recognizing metastases to the breast from an extramammary location. The morphologic evaluation of the tissue stained with hematoxylin and eosin on microscopic examination may offer pertinent clues, such as atypical morphology for a primary breast carcinoma or pigment within the tissue, possibly suggesting a melanoma. Immunohistochemistry is a powerful tool in helping to distinguish between a primary breast carcinoma and a metastatic tumor from an extramammary location. Molecular profiling has promising potential in the workup of a carcinoma in the breast which is suspected to be of metastatic origin. This chapter will review the current literature on the epidemiology, differential diagnosis, clinical presentation, and diagnostic workup of metastatic cancer to the breast.