MicroRNAs(miRNAs), small noncoding RNAs, are essential for posttranscriptional gene regulation and have important roles in a wide range of biological processes, including development, growth and differentiation. Thousands of miRNAs have been identified in animals, plants and microoaganisms. miRNAs regulate their target genes by mRNA degradation or translation suppression. About 30% of genes in an organism are subject to miRNA regulation. miRNA expression and function are regulated by transcriptional factors, epigenetics, single nucleotide polymorphisms, RNA editing and so on. Additionally, the success in knocking out a specific mouse miRNA gene has provided a valuable model for studying miRNA function.