Antisense RNA control is now recognized as an efficient and specific means of regulating gene expression at the posttranscriptional level. Almost all naturally occurring cases have been found in prokaryotes, often in their accessory genetic elements. Several antisense RNA systems are now well-understood, and these display a spectrum of mechanisms of action, binding pathways, and kinetics. This review summarizes antisense RNA control in prokaryotes, emphasizing the biology of the systems involved.