RNA interference (RNAi) is a mechanism that inhibits gene expression by causing the degradation of specific RNA molecules or hindering the transcription of specific genes. RNAi targets include RNA from viruses and transposons (probably as a form of innate immune response), and also plays a role in regulating development and genome maintenance. Small interfering RNA strands (siRNA) are key to the RNAi process, and have complementary nucleotide sequences to the targeted RNA strand. Specific RNAi pathway proteins are guided by the siRNA to the targeted messenger RNA (mRNA), where they cleave the target, breaking it down into smaller portions that can no longer be translated into protein. A type of RNA transcribed from the genome itself, microRNA (miRNA), and works in the same way. This book presents the latest research in the field from around the globe.