The onset of puberty is known to be regulated by a complex network of genes; however, our understanding lacks mechanistic details. In this paper, the authors found that puberty in rats is preceded by increased promoter DNA methylation and downregulation of two Polycomb genes in the hypothalamus. These Polycomb genes were found to repress the puberty-activating gene Kiss1, and inhibition of DNA methylation resulted in their sustained expression and pubertal failure. Hence, the controlled repression of Polycomb genes in the hypothalamus is an epigenetic mechanism for regulating the timing of puberty.