Steroid action is mediated by specific intracellular receptors, which are shifted to a transcriptionally active state after ligand binding. In 1996, the cloning of a new member of the nuclear receptor superfamily from the rat prostate was reported. Ligand-binding experiments have shown that this receptor binds specifically to oestrogens and it has been named oestrogen receptor beta (ER beta) to distinguish it from the oestrogen receptor (ER alpha) cloned from uterus in 1986. The alpha and beta forms of the oestrogen receptor have identical numbers of exons, and the cDNAs cloned from humans, rats and mice all share significant sequence homologies especially within their DNA and ligand-binding domains. Splice variants of ER beta have been identified. ER beta mRNA and protein have been detected in a wide range of tissues including the vasculature, bone, brain, heart and the gonads and genital tracts in both males and females, and in some, but not all, tissues the pattern of expression is distinct from that of ER alpha. Studies in vitro have demonstrated that ER alpha and ER beta can exist as hetero- or homodimers and that these forms may interact differentially with response elements on genes. The identification of ER beta has made us rethink the potential sites of action of both endogenous oestrogens and exogenous natural and synthetic oestrogens and anti-oestrogens and is currently the subject of intensive research efforts.