The biological actions of androgens are mediated by transcriptional control of target genes through binding to the androgen receptor (AR). The AR belongs to the nuclear receptor superfamily and acts as a ligand-inducible transcriptional factor. Androgens/AR system is thought to be central to male physiology and behaviors to the development of clinical pathology. The physiological importance of AR function currently recognized in female reproduction also has been verified in mouse genetic model. Here we review functions of AR in male and female physiology as revealed by mice lacking AR.