Intensive treatment regimens now result in long-term survival of more than 70% of children with cancer. This has come to the cost of late adverse effects among which infertility is a major burden. There are thus urgent needs of novel strategies for fertility preservation. In animals fertility can be restored by autotransplantation of cryopreserved testicular cells into the testis or by xenografting of testicular tissue into intermediate host animals to retrieve functional sperm. However, the safety of such approaches has not been well investigated. This study evaluated the putative risks with use of testis tissue from leukemic donors for transplantation, potential production of sperm from such grafts, and the possibility of eliminating contaminated leukemic cells in testicular samples. Advances in the field of fertility preservation, future research directions and how to retrieve testicular tissue by biopsy before cancer treatment are discussed. This book provides an overview for pediatric oncologists and basic knowledge for scientists and a lay audience with interest in the field.