Research on the cytochrome P450 family of genes has traditionally been dominated by forms participating in drug metabolism. This has occurred in spite of early discovery of steroid hydroxy lase P450 cytochromes in the adrenal gland. More recently, contributions on the characterization and regulation of P450 cytochromes involved in biosynthetic reactions have been found at the international meetings on cytochrome P450 and in the several books on the field. Key recognition that P450 cytochromes should be recognized in a physiological context was provided by an international meeting in Jerusalem in 1991 and the subsequent publication of the proceedings in the Journal of Steroid Biochemistry and Molecular Biology (43, number 8, 92). Like this meeting, this book seeks to place equal weight on the physiological processes that are controlled by the products of reactions at usually very selective cytochrome P450 forms. Each of the authors was asked to discuss the molecular regulation of these P450 forms. Each of the authors was asked to discuss the molecular regulation of these P450 forms in the light of these physiological processes. In some cases the physiological role of the cytochrome P450 and even the natural substrate are unresolved, but a pattern of strong endocrine regulation is indicative of a hidden function. As more and more low abundance P450 genes are uncovered, the need to address potential physiological activities becomes more pressing. It is almost infinitely more difficult to identify a physiological substrate than to clone a new form.