Cancer is more common after puberty during the reproductive life span of men and women, and many of the patients will be cured by combination treatment with surgery, chemotherapy and radiotherapy. The model of human ovarian reserve allows us to estimate the number of non-growing follicles (NGFs) present in the ovary at any given age, and it suggests that 81% of the variance in NGF populations is due to age alone. The ovary is susceptible to chemotherapy-induced damage, particularly following treatment with alkylating agents such as cyclophosphamide. Ovarian damage is drug- and dose-dependent and is related to age at the time of treatment, with progressively smaller doses required to produce ovarian failure with increasing age. Preventing chemotherapy-induced damage to the ovary or testes remains an elusive ideal in the field of fertility preservation. Both chemotherapy and radiotherapy can impair future fertility, and treatments for certain cancers can be sterilizing.