The use of double-stranded (ds) RNA to manipulate gene expression is evolving into an increasingly powerful tool to study gene function in organisms ranging from plants to invertebrates to mammals. Underlying the technological advances enabled by RNA interference (RNAi) is a rich biology. RNAi and related phenomena represent evolutionarily conserved mechanisms, which protect organisms from invasion by both exogenous (e.g., viruses) and endogenous (e.g., mobile genetic elements) genetic parasites. In addition, it is now clear that dsRNA-dependent silencing mechanisms represent a conserved regulatory motif for endogenous programs of gene expression. We are just beginning to understand not only the biological roles of RNAi but also the mechanistic basis of this process. The goal of this review is to familiarize the reader with the diversity of biological responses that may be related to RNA interference and to summarize the current state of knowledge regarding the mechanistic bases of these silencing phenomena.