Aromatase is the enzyme that converts androgen to estrogen. Our laboratory has proposed and demonstrated that aromatase is an important target of some environmental chemicals. We have found that some of these compounds inhibit aromatase activity, resulting in a decrease in the level of estrogen or an increase in the level of androgen in cells. Environmental chemicals can also modify the expression of aromatase in various tissues, resulting in a change in the ratio between androgen to estrogen. The compounds that inhibit aromatase or suppress aromatase expression will behave as antiestrogens or androgen-like compounds in vivo. On the other hand, compounds that increase aromatase expression or enhance aromatase activity (or stability) may function as anti-androgens or estrogen-like compounds. This review will summarize and discuss findings from this and other laboratories on the effects of environmental chemicals on aromatase activity and expression.