Estrogen receptor (ER)- (alpha, beta, and both alpha and beta) and aromatase (Ar) knockout (KO) mice have been created to assess the biological effects of estrogens. This review article discusses the ovarian and uterine phenotypes of these mice. The data obtained have confirmed some older inferences about how the steroid acts, but have also revealed some unexpected aspects of estrogen action. (C) 2001 Wiley-Liss, Inc.