The importance of small, noncoding RNAs that act as regulators of transcription, of RNA modification or stability, and of mRNA translation is becoming increasingly apparent. Here we discuss current knowledge of regulatory RNA function and review how the RNAs have been identified in a variety of organisms. Many of the regulatory RNAs act through base-pairing interactions with target RNAs. The base-pairing RNAs can be grouped into two general classes: those that are encoded on the opposite strand of their target RNAs such that they contain perfect complementarity with their targets, and those that are encoded at separate locations on the chromosome and have imperfect base-pairing potential with their targets. Other regulatory RNAs act by modifying protein activity, in some cases by mimicking the structures of other RNA or DNA molecules.