Here we summarise the results that have emerged from our structural studies on the oestrogen receptor (ER) ligand-binding domain. We have investigated the conformational effects of a variety of ligands on the structures of both ER isoforms. Each class of ligand (agonists, partial agonists and selective oestrogen receptor modulators) induces a unique conformation in the receptor's ligand-dependent transcriptional activation function. Together these studies have broadened our understanding of ER function by providing a unique insight into ER's ligand specificity and the structural changes that underlie receptor agonism and antagonism.