Abstract:piRNAs are a germline specific class of small non-coding RNAs. They associate with PIWI proteins which belong to a family of Agonaute proteins. piRNA and PIWI proteins form piRNA-induced silencing complexes (piRISC)， and function in defending genome integrity by TE silencing. Recent studies have revealed a large number of factors that are essential for piRNA biogenesis， piRISC formation and piRNA-mediated transcriptional silencing. This review summaries these key factors involved in piRNA pathways of Drosophila melanogaster and mouse.