For the last two decades, the use of bone marrow transplantation for leukaemia, and most recently for lymphomas and other cancers, has increased dramatically and all of the systemic chemotherapy or radiotherapy used in these patients has a potential for causing infertility. This review describes the normal reproductive function of relevance to patients at risk of chemotherapy- or radiotherapy-induced infertility. The mechanism by which fertility may be affected by these agents is also described and therapeutic strategies for reducing or preventing these problems are outlined. The treatment of men and women who are infertile after chemotherapy or radiotherapy is addressed, as are the ethical and legal aspects of assisted fertilization.