Non coding endogenous RNAs were first discovered in the last decade of the previous century. These new discoveries changed our views of the transcriptome landscape of plant genomes and paradigms of the regulation of gene expression. With the beginning of this century, we have witnessed an explosion of studies on small regulatory RNAs that has yielded a basic understanding of the many types of small RNAs in diverse eukaryotic species and how they are functioning as RNA–protein complexes along the RNA silencing pathways. While reading this book, the reader will realize that much more remains to be learned about the non coding RNAs and their complex regulatory mechanisms, and.