One of the main prognostic markers measured in breast tumours is oestrogen receptor (ER) status. With the discovery of ER beta, it has been necessary to re-evaluate ER signalling and the role of ER in breast cancer. Preliminary reports on ER beta signalling suggest it might induce opposite effects to those of ER alpha. To further understand the biology of breast cancer, the role each ER plays in disease progression must be established. This review summarizes the current understanding of ER beta and discusses some of the published work on the role of ER alpha and ER beta in breast cancer.