Recent advances in the treatment of teenage cancers have led to an increasing focus on the late effects of treatment, amongst which fertility is prominent with surveys ranking potential loss of fertility as amongst the most important concerns of teenage and young adult patients. The various treatments for teenage cancer including chemotherapy, radiotherapy and surgery can all potentially compromise fertility with the most toxic therapies being alkylating agent based chemotherapy and pelvic irradiation. Assessment at the time of diagnosis and prior to starting treatment is therefore critical in providing patient information and suggesting potential fertility preservation strategies, where relevant. Sperm cryopreservation is well established but prediction of spermarche is difficult. Cryopreservation of testicular tissue is being developed but remains experimental. For girls, ovarian tissue cryopreservation may be an option and for older teenage girls ovarian stimulation for oocyte vitrification may also be possible, although there is limited experience in developing appropriate protocols for such patients. It is important to recognise that most teenagers treated for cancer will retain their fertility and thus for many, assessment and reassurance are appropriate. Following treatment, conventional semen analysis provides the best predictor of male fertility but some young men may well be reluctant to have this investigated. In young women, premature ovarian insufficiency (POI) provides a clear indicator of loss of fertility, and in those with ongoing ovarian activity the key question may be prediction of the fertile lifespan. There is also evidence for a significant prevalence of infertility in women even with maintained ovarian activity following cancer therapy. Radiotherapy to the uterus carries a substantial risk of miscarriage or obstetric complications and pregnancy in such women should be managed in a high risk obstetric unit. Pregnancy following egg donation should also be regarded as high risk. There is increasing interest in the use of serum anti-Müllerian hormone to predict reproductive lifespan, but its ability to predict fertility in young women is probably very limited.