I. Introduction IT HAS been known for several years that steroid hormones function through the action of specific receptor proteins. In target tissues, receptors are activated by hormonal ligands and thereafter modulate the expression of a network of specific target genes (1–5). Receptors for the steroids, glucocorticoid, androgen, estrogen, progesterone, and mineralocorticoid, and the sterol, 1,25-dihydroxyvitamin D3, have been identified and characterized by biochemical and molecular biological methods (6–10). Other nonsteroid, seemingly unrelated molecules such as T4 and retinoic acid have been found to have receptors that are structurally related to steroid receptor hormones (11–13) and appear to act through similar mechanisms as the steroid receptors. The mechanism by which receptors function from the time of the initial appearance of hormone, to resultant effects at the gene level, to the cessation of the hormone effect, must be complex. Although there may be a general pathway of receptor function, selective differences must exist in the pathway of action of different receptor molecules which produce the effects unique to their particular hormones. In this review, we will summarize recent information on the functional pathway whereby steroid hormones mediate their effects on cellular function and development. We will stress certain aspects of progesterone receptor action, notably PR synthesis, but will attempt to present a balanced view of the salient features of all receptors in the superfamily. The similarities and differences in the various receptors will be summarized, and for issues that remain unresolved, opposing theories will be presented.