This book focuses on an emerging, central issue in molecular genetics and the development of eukaryotes: the control of gene expression by small species of RNA. As an exciting new field of endeavor, it is the first book by a single author to deal comprehensively with RNA silencing.The book provides the historical background of the field preceding the seminal work by Fire and associates in 1998 on the impact of small double-stranded RNA on the expression of nematode genes, which is considered the beginning of RNA silencing research. RNA silencing is described in a wide range of plants and animals including protozoa, simple metazoa, insects, non-mammalian vertebrates, and mammals. In each case the experimental results are provided with the accompanying background and with illustrations. There is also an appendix on the prospective use of RNA silencing in gene therapy, which is intended as a guide for investigators wishing to explore this possibility.