As the proper development of germlines is vital for species preservation, elaborative, regulatory systems for gene expression must operate in germlines. One such system is RNA silencing, sequence-specific gene silencing mechanisms mediated by small RNAs of 20-30 nucleotides long. Indeed, recent studies have revealed that various types of small RNAs are expressed germline-specifically. To preserve the germlines, they collaborate with Argonaute proteins, the catalytic engines in RNA silencing, to inhibit injurious, parasitic genes, transcriptionally or post-transcriptionally. This chapter summarizes the exquisite collaboration of Argonaute proteins with small RNAs in the RNA silencing mechanisms necessary for germline survival in Drosophila and mice.