Mobilization of transposable elements can lead to natural insertion mutations that often have negative effects on the host genome. Thus, host species have evolved control mechanisms that restrict the transposon activity. One such mechanism is RNA silencing, in which small RNAs of 20–30 nucleotides (nt) trigger multiple forms of sequence-specific gene silencing. In the germline, PIWI-interacting RNAs (piRNAs) are produced from single-stranded long non-coding RNAs that are transcribed from intergenic piRNA clusters on the genome. Mature piRNAs are loaded onto PIWI proteins to yield piRISCs and guide the complexes to target RNAs to silence them. piRNAs in the germ cells are amplified by the cytoplasmic “ping-pong” cycle, in which transposon transcripts are consumed as both the source of piRNAs and the targets of PIWI cleavage, thereby repressing transposons in the cytoplasm. It is becoming clear that piRISCs also mediate transposon silencing in the nucleus in a PIWI cleavage-independent manner. However, the underlying mechanism remains unknown. We are currently focusing on piRNA biogenesis to understand the underlying mechanisms. The outcome is discussed at the meeting.