For a long time, RNA has been merely regarded as a molecule that can either function as a messenger (mRNA)or as part of the translational machinery (tRNA, rRNA). Meanwhile, it became clear that RNAs are versatile molecules that do not only play key roles in many important biological processes like splicing, editing, protein export and others, but can also-like enzymes-act catalytically. Two important aspects of RNA function-antisense-RNA control and RNA interference (RNAi)-are emphasized in this review. Antisense-RNA control functions in all three kingdoms of life-although the majority of examples are known from bacteria. In contrast, RNAi, gene silencing triggered by double-stranded RNA, the oldest and most ubiquitous antiviral system, is exclusively found in eukaryotes. Our current knowledge about occurrence, biological roles and mechanisms of action of antisense RNAs as well as the recent findings about involved genes/enzymes and the putative mechanism of RNAi are summarized. An interesting intersection between both regulatory mechanisms is briefly discussed.