Repetitive genomic elements called transposons are capable of mobility within the genome and are a potential source of mutations causing genome instability. Mobilization within the germline is particularly advantageous for the transposons, as it can be inherited, allowing them to spread in the population. Animal germlines have evolved a dedicated transposon defense mechanism, and central to this is a class of 24-30 nucleotide (nt) long small RNAs called Piwi-interacting RNAs (piRNAs). Argonautes are proteins specialized in binding small RNAs, and Piwis are gonad-specific members of this family. In its simplistic form, piRNAs guide Piwi proteins to target transposon transcripts for endonucleolytic (Slicer) cleavage in the cytoplasm. In Drosophila and mice, nuclear Piwi proteins are believed to recruit repressive heterochromatin or DNA methylation marks on transposon loci, leading to transcriptional repression. Recent findings on this topic will be discussed