Ovarian steroids, acting through nuclear receptors, are crucial players in normal breast development and cancer. Estrogen, in particular, is the focus of breast cancer therapies because tumours are often dependent on this steroid for growth. Recently, novel genes and/or protein isoforms of receptors for both estrogen and progesterone have been discovered, leading us to reappraise their roles in breast development and cancer. Recognition of changes in estrogen receptor biology that occur in the transition from normal development to cancer has emphasized its contribution to tumorigenesis. In addition, complex interactions with other signalling pathways, particularly growth factor pathways, have recently come to the forefront. These interactions might explain resistance to endocrine treatments and offer solutions in terms of novel therapeutic targets.