microRNAs (miRNAs) and small interfering RNAs (siRNAs) play important roles in gene regulation and defense responses against transposons and viruses in eukaryotes. These small RNAs generally trigger the silencing of cognate sequences through a variety of mechanisms, including RNA degradation, translational inhibition and transcriptional repression. In the past few years, the synthesis and the mode of action of miRNAs and siRNAs have attracted great attention. However, relatively little is known about mechanisms of quality control during small RNA biogenesis as well as those that regulate mature small RNA stability. Recent studies in Arabidopsis thaliana and Caenorhabditis elegans have implicated 3′-to-5′ (SDNs) and 5′-to-3′ (XRN-2) exoribonucleases in mature miRNA turnover and the modulation of small RNA levels and activity. In the green alga Chlamydomonas reinhardtii, a nucleotidyltransferase (MUT68) and an exosome subunit (RRP6) are involved in the 3′ untemplated uridylation and the degradation of miRNAs and siRNAs. The latter enzymes appear to function as a quality control mechanism to eliminate putative dysfunctional or damaged small RNA molecules. Several post-transcriptional modifications of miRNAs and siRNAs such as 3′ terminal methylation and untemplated nucleotide additions have also been reported to affect small RNA stability. These collective findings are beginning to uncover a new layer of regulatory control in the pathways involving small RNAs. We anticipate that understanding the mechanisms of mature miRNA and siRNA turnover will have direct implications for fundamental biology as well as for applications of RNA interference technology.