RNA interference (RNAi) is the sequence-specific gene silencing induced by double-stranded RNA (dsRNA). Being a highly specific and efficient knockdown technique, RNAi not only provides a powerful tool for functional genomics but also holds a promise for gene therapy. The key player in RNAi is small RNA (~22-nt) termed siRNA. Small RNAs are involved not only in RNAi but also in basic cellular processes, such as developmental control and heterochromatin formation. The interesting biology as well as the remarkable technical value has been drawing widespread attention to this exciting new field.