Clinical management of breast cancer is increasingly guided by assessment of tumor phenotypic parameters. One of these is estrogen receptor (ER) status, currently defined by ERα expression. However with the discovery of a second ER, ERβ and its variant isoforms, the definition of ER status is potentially more complex. In breast tumors there are two ERβ expression cohorts. One where ERβ is co-expressed with ERα and the other expressing ERβ alone. In the latter subgroup of currently defined ER negative patients ERβ has the potential to be a therapeutic target. Characterization of the nature and role of ERβ in ERα negative tumors is essentially unexplored but available data suggest that the role of ERβ may be different when co-expressed with ERα and when expressed alone. This review summarizes available data and explores the possibility that ERβ signaling may be a therapeutic target in these tumors. Evidence so far supports the idea that the role of ERβ in breast cancer is different in ERα negative compared to ERα positive tumors. However, cohort size and numbers of independent studies are small to date, and more studies are needed with better standardization of antibodies and protocols. Also, the ability to determine the role of ERβ in ERα negative breast cancer and therefore assess ERβ signaling pathways as therapeutic targets would be greatly facilitated by identification of specific downstream markers of ERβ activity in breast cancer.