In the last decade or so, our knowledge of the roles of the steroids referred to as ‘sex hormones’ namely testosterone and estradiol, has undergone a quiet revolution. In the first place, models of androgen and estrogen insufficiency, whether they be natural or engineered, have revealed new and unexpected roles for sex steroids, some of which have nothing to do with reproduction. Furthermore both categories of steroid have roles in both sexes which blunts the definition of the terms ‘androgen’ and ‘estrogen’. Secondly, the gradual acceptance of the role of local steroid hormone action, particularly as it applies in postmenopausal women and in men, provides new insights into the significance of paracrine and intracrine actions, and requires a re-evaluation of the importance of circulating steroid hormone levels. This article will attempt to review both of these developments, particularly in the context of the work of this laboratory on the aromatase knock-out (ArKO) mouse and the role of local aromatase expression within the breast and breast cancer.