The discovery of a second oestrogen receptor, ER beta, was a subject of much interest, as this suggested a means to improve the prognostic stratification of invasive breast cancer, better predict response to endocrine therapy, develop new chemotherapeutic/chemopreventative drugs and perhaps prevent inappropriate treatment. However, this has not proved to be straightforward with the discovery of five ER beta isoforms and numerous exon deletion variants. This review sets out to identify the present state of knowledge regarding the clinicopathological role of ER beta isoforms and discusses possible reasons for conflicting results arising from recent research findings.