Spermatogenesis is the process of sperm production that occurs within the testes in mammals. This is a tremendously productive cell lineage, resulting in production of millions of functional sperm every day (a man produces 1000s of sperm every time his heart beats). This remarkable process involves mitotic amplification of differentiating spermatogonia from a pool of spermatogonial stem cells, meiotic production of haploid germ cells, and morophological differentiation of spermatids throuh spermiogenesis. The stem cell pool is essential for this process and is maintained through balanced self-renewal and differentation to ensure adequate and sustained production of spermatogonia to meet the biological demand of sperm production. We study how spermatogonial stem cells are regulated to perform these critical biological functions.