The tendency to delay childbirth has increased the importance of ovarian reserve as a determinant of infertility treatment outcome. In the context of assisted reproduction technology, effective strategies to overcome the impact of ovarian aging and diminished ovarian reserve on pregnancy chances remain elusive. Markers of ovarian reserve are increasingly used to aid management and counseling of these patients. Proper interpretation of currently applied hormonal markers, ultrasound parameters, and hormone challenge tests requires an understanding of what constitutes and determines ovarian reserve. This article addresses these aspects and highlights recent developments in the field.