The role of oestrogen receptor (ER) beta in human breast cancer remains unclear. However, it is now apparent that when considering ER beta in human breast cancer it is important to recognise two ER beta expressing groups, one in which ER beta is co-expressed with ER alpha and the other where ERbeta is expressed alone. Emerging data support different functions between ER beta when it is expressed alone and when it is co-expressed with ER alpha. With regard to the latter group (ER alpha +/ER beta +), there are now 9 out of 10 retrospective clinical outcome studies published, that support the hypothesis that increased expression of ER beta is associated with increased likelihood of response to endocrine therapy. The data strongly support undertaking prospective studies to determine if the addition of ERbeta to ER alpha is clinically beneficial and whether to include both ER beta and ER alpha when establishing clinically relevant cut-offs for defining ER status.