Decades of study have described a number roles fulfilled by the steroid hormones and their respective receptors in sexual differentiation and development, reproductive function and behavior, and more recently in the function and maintenance of non-reproductive organ systems, such as skeletal muscle, bone and coronary tissues. The biological effects of the steroid hormones are believed to be mediated in part by specific receptor proteins that demonstrated great specificity for their respective steroid ligands. Much of the experimental research of the functions of the sex steroid receptors has depended upon in vitro systems as well as in vivo methods that require surgical castration or the pharmacological administration of hormone antagonists. However, recently developed techniques that allow for manipulation of the mouse genome have been utilized to generate transgenic animals that lack functional estrogen or progesterone receptors. These transgenic animals, combined with the naturally existing Tfm mice which lack functional androgen receptor, now provide in vivo models for further study of the various actions of the sex steroids and their receptors. This review attempts to describe and compare the various phenotypes that result in each of these lines of mice, with emphasis on the development and function of the reproductive systems as well as reproductive behavior.