Increasingly young people survive cancer in childhood and as a result complications of its treatment are becoming more common and important. Premature ovarian failure is recognized as a complication of radiotherapy to a field that includes the pelvis and alkylating-agent-based chemotherapy. Young pre-pubertal girls are not protected from the effects of gonadal toxic therapy. A young woman, successfully treated for cancer during childhood, may experience regular periods in the presence of a significantly reduced ovarian reserve. There is, however, no reliable measure of ovarian reserve available for the individual woman. Assessment of ovarian function relies on the use of surrogate markers such as follicle stimulating hormone, inhibin-B, and anti-mullerian hormone as well as ultrasound assessment of ovarian volume and antral follicle count. We discuss the physiology of normal ovarian function, the effects of cancer treatments on ovarian function and the techniques for evaluation of ovarian reserve in survivors of childhood cancer. Pediatr Blood Cancer 2009;53:296–302. © 2009 Wiley-Liss, Inc.