In many species, germline stem cells (GSCs) function to sustain gametogenesis throughout the life of organismal life span. As the source of gametes, the only cell type that can pass the genetic information to the next generation, GSCs play a fundamental role in maximizing the quantity of gametes that animals produce, while ensuring their highest quality. GSCs are maintained by the signals from their niches, and germ cells that exited the niche undergo differentiation to generate functional gametes. GSC population is sustained by a multitude of mechanisms such as asymmetric stem cell divisions and dedifferentiation of partially differentiated germ cells. In this review, we summarize the mechanisms that maintain GSC homeostasis to ensure life-long production of functional gametes.