In this review, we have traced the path of estradiol from its entry into the cell to the time of its release from the receptor. We feel that all of the current models are limited in one respect or another. We have examined most critically those currently most in vogue. The entry of estradiol into the cell is widely assumed to be simply a matter of diffusion. We have highlighted data that suggest the existence of a protein-mediated transport, but feel that the available data are too limited to make a definite conclusion.