Male germ-cell maturation is orchestrated by a cascade of temporally regulated factors. Gonadotropin-regulated testicular helicase (GRTH/DDX25), a target of gonadotropin and androgen action, is a post-transcriptional regulator of key spermatogenesis genes. Male mice lacking GRTH are sterile, with spermatogenic arrest owing to the failure of round spermatids to elongate. GRTH is a component of messenger ribonucleoprotein particles, which transport target mRNAs to the cytoplasm for storage in chromatoid bodies of spermatids; these messages are released for translation during spermatogenesis. GRTH is also found in polyribosomes, where it regulates the translation of mRNAs encoding spermatogenesis factors. The association of GRTH mutations with male infertility underlines the importance of GRTH as a central, post-transcriptional regulator of spermatogenesis.