Despite signiﬁ cant advances in the diagnosis and treatment of breast cancer, approximately one-third of patients still develop and subsequently die from metastatic breast disease. Globally, half a million deaths each year are attributable to metastatic breast cancer and the median survival time from the diagnosis of secondary disease is approximately 3 years. The range is very wide however, with some patients having more indolent disease that they can live with for 10-15 years, while for others with widespread metastatic disease, the prognosis may only be a matter of months from the time of diagnosis. While this may represent the extent and distribution of metastatic disease, in part it reﬂ ects the biological diversity of breast cancer, with some women having disease that exhibits extreme sensitivity to hormonal treatments, whereas others with so-called triple-negative breast cancer may display relative resistance to all systemic therapies. In recent years, the increasing recognition that different molecular subtypes of breast cancer exist has substantially changed not only the way we classify and treat the disease, but also the impact that certain novel therapeutics in the metastatic setting can have on speciﬁ c types of breast cancer.