The number of men surviving cancer at a young age has increased dramatically in the past 20 years as a result of early detection and improved cancer treatment protocols; more than 75% of young cancer patients nowadays are long-term survivors. Quality of life has become an important issue in childhood and adult cancer patients. The commonest cancers in patients of reproductive age are leukaemia, Hodgkin's lymphomas and testicular germ cell tumors. Fertility is often impaired after chemotherapy and radiation therapy. Cryopreservation of semen before cancer treatment starts is currently the only method to preserve future male fertility. In some malignancies, especially in germ cell tumors, sperm quality is already abnormal at the time of diagnosis. In approximately 12% of men, no viable spermatozoa are present for cryopreservation before the start of chemotherapy. Cytotoxic therapy influences spermatogenesis at least temporarily and in some cases permanently. The amount of damage inflicted by chemotherapy on spermatogenesis depends on the combination of drugs used and on the cumulative dose given for cancer treatment. Alkylating agents, such as cyclophosphamide and procarbazine, are most detrimental to germ cells. Radiation therapy, especially whole-body irradiation, is also associated with the risk of permanent sterility. Besides the cancer treatment, tumor type and pretreatment fertility are of prognostic value for future fertility in male cancer survivors. After cancer treatment, many men need artificial reproductive techniques to achieve fatherhood; usually in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI) is indicated for successful treatment. About 15% of men will use their cryopreserved semen because of persistent azoospermia after cancer treatment. Treatment results with cryopreserved semen are generally good and comparable to general IVF and ICSI results. So far, no studies have reported an increased rate of congenital abnormalities or malignancies in children born from fathers who had cancer treatment is the past, but close follow up is warranted, especially in children born after IVF/ICSI.