MicroRNAs (miRNA) belong to a novel large family of short single-stranded conserved noncoding regulatory RNAs that include the small temporal RNAs lin4 and let7. They exhibit a diversity in sequence, structure, abundance, and expression profile. There are similarities and differences between miRNAs and another class of RNAs called short interfering RNAs (siRNA). The recent advance of miRNAs is reviewed.