Spermatogenesis involves an early wave of germ cell apoptosis, which is required for maintaining the balance between germ cells and supporting Sertoli cells. However, the signaling mechanism regulating this apoptotic event is poorly defined. Here we show that genetic deficiency of Cyld, a recently identified deubiquitinating enzyme, attenuates the early wave of germ cell apoptosis and causes impaired spermatogenesis in mice. Interestingly, the loss of CYLD in testicular cells leads to activation of the transcription factor NF-kappaB and aberrant expression of antiapoptotic genes. We further show that CYLD negatively regulates a ubiquitin-dependent NF-kappaB activator, RIP1. CYLD binds to RIP1 and inhibits its ubiquitination and signaling function. These findings establish CYLD as a pivotal deubiquitinating enzyme (DUB) that regulates germ cell apoptosis and spermatogenesis and suggest an essential role for CYLD in controlling the RIP1/NF-kappaB signaling axis in testis.