Pharmacokinetic variability of contraceptive steroids is a relatively under-explored area of contraceptive research, and hardly a common point of discussion among those who plan and deliver family planning services. Nevertheless, numerous independent studies over the last 15 years have indicated that women in different regions of the world vary in their pharmacokinetic response to contraceptive steroids. The causes of such variability are not known, but it has important consequences for contraceptive effectiveness. It may also offer insight to the basis of contraceptive side-effects. The impetus for this volume was to collect documentation of pharmacokinetic variability of contraceptive steroids, and to explore both the possible causes and implications of these data. Factors known to affect steroid pharmacokinetics, such as concurrent use of specific medications, are reviewed by Back and Orme. Other factors known to affect endogenous steroid dynamics are presented in chapters by Bradlow, Longcope, Goldin and Snow, because of their possible role in contraceptive steroid pharmacokinetics.