The diverse effects of progesterone on female reproductive tissues are mediated by the progesterone receptor (PR), a member of the nuclear receptor family of ligand-dependent transcription factors. Thus, PR is an important therapeutic target in female reproduction and in certain endocrine dependent cancers. This paper reviews our understanding of the mechanism of action of the most widely used PR antagonist RU486. Although RU486 is a competitive steroidal antagonist that can displace the natural hormone for PR, it's potency derives from additional "active antagonism" that involves inhibiting the activity of PR hormone agonist complexes in trans through heterodimerization and competition for binding to progesterone response elements on target DNA, and by recruitment of corepressors that have the potential to actively repress gene transcription. An additional functional role for PR has recently been defined whereby a subpopulation of PR in the cytoplasm or cell membrane is capable of mediating rapid progesterone induced activation of certain signal transduction pathways in the absence of gene transcription. This paper also reviews recent results on the mechanism of the extra-nuclear action of PR and the potential biological roles and implications of this novel PR signaling pathway.