The preservation and transplantation of spermatogonial stem cells (SSCs) is a potential strategy to circumvent sterility: frozen-thawed SSCs can be transplanted back into the cured patient to restore spermatogenesis. For almost 20 years, the research group BITE performed pioneering work to develop this strategy towards a clinical application. For patients at high risk of losing their SSCs, the cryopreservation of SSCs is a valuable clinical tool to preserve their fertility. The research group BITE developed optimal protocols to cryopreserve either testicular cell suspensions or testicular tissue containing SSCs. These protocols are now being used in clinical fertility preservation programs. The UZ Brussel started in 2002 to store testicular tissue of young boys who are at high risk of losing their spermatogonia. In order to harvest their testicular tissue, these young boys need to undergo a testicular tissue biopsy. Although the biopsy procedure itself does not have a high complication rate, little is known about its effects on the long-term. BITE studies the effect of a testicular biopsy on the later pubertal development and fertility.