Progesterone, supplied in a variety of formulations, is commonly used in assisted reproduction. However, even the commonly accepted uses of progesterone in fertility care are believed to be largely empirical, as are the current available treatment options. While a critical balance of estrogen and progesterone is necessary for successful embryo implantation, achieving that balance is subject to differences in prescribing practice. In evaluating recommendations for the use of progesterone, discussion of its role in assisted-reproduction procedures must distinguish between known therapeutic benefits and empirical benefits.