Gene silencing is a general term describing epigenetic processes of gene regulation. The term gene silencing is generally used to describe the 'switching off' of a gene by a mechanism other than genetic mutation. That is, a gene which would be expressed (turned on) under normal circumstances, is switched off by machinery in the cell. Genes are regulated at either the transcriptional or post-transcriptional level. Transcriptional gene silencing is the result of histone modifications, creating an environment of heterochromatin around a gene that makes it inaccessible to transcriptional machinery (RNA polymerase, transcription factors, etc.). Post-transcriptional gene silencing is the result of mRNA of a particular gene being destroyed. The destruction of the mRNA prevents translation to form an active gene product (in most cases, a protein). A common mechanism of post-transcriptional gene silencing is RNAi. Both transcriptional and post-transcriptional gene silencing are used to regulate endogenous genes. This book presents the latest research in this important field.