Antisense RNA-mediated regulation is widespread in bacteria. Most antisense RNA control systems have been found in plasmids, phages, and transposons. Fewer examples were identified in bacterial chromosomes. This chapter summarizes our current knowledge about antisense RNAs with respect to their occurrence, their biological roles, and their diverse mechanisms of action. Examples of cis- or trans-encoded antisense RNAs are discussed, and their properties compared. Most antisense RNAs are posttranscriptionally acting inhibitors of target genes, but a few examples of activator antisense RNAs are known. The implications of RNA structure on topologically and kinetically favored binding pathways are addressed, and solutions that have evolved to permit productive interactions between intricately folded RNAs are discussed. Finally, we describe how particular properties of individual antisense/target RNA systems match their respective biological roles.