"In many European countries at least 20% of young men exhibit sperm parameters below the lower WHO reference level and this will affect their fertility. Male infertility has a dramatic impact on the individual and couple’s psychological and social well-being and results in significant healthcare costs. Currently male patients that do not produce sperm have no therapeutic options to father children.Different therapeutic interventions for male infertility have to be developed depending on the severity of germ cell deficits in individual patients. In cases where undifferentiated germ cells are present in the testis, strategies based on sperm development from spermatogonial stem cells (SSCs) in vitro or in vivo need to be established. However, if no germ cells are present in the testis, somatic cells of such patients will be the only option from which to develop their own sperm. Development of these potential therapies requires detailed understanding of the entire process of sperm production from stem cells through to functional sperm. This information is still incomplete and fragmented.The current proposal seeks to train young scientists in a network that joins together the complementary knowledge and expertise of several public and private EU partners from disciplines of physiology, cell biology, molecular biology, chemistry and medicine in the field of male reproduction to investigate three strategies for sperm development: (1) propagation of human SSCs in vitro followed by their transplantation, (2) sperm development in vitro from stem cells or early germ cells, (3) sperm development in human testis tissue grafts in vivo.By coaching young scientists in this inter-sectorial and multidisciplinary network, we will train the next generation of researchers within the EU who are then primed to become leaders in the field of male fertility that continue to investigate basic science and translational aspects leading to novel interventions and clinical applications."