RNA switches (riboswitches) have important functions in gene regulation. They comprise an aptamer domain, which is responsible for ligand binding, and an expression platform that transmits the ligand-binding state of the aptamer domain through a conformational change. Riboswitches can regulate gene expression either at the level of transcription or translation, and it has been proposed that riboswitch mechanisms are even used to regulate the processing of mRNA. This Minireview summarizes the current understanding of the structures and mode of action of RNA switches, with particular focus on secondary and tertiary interactions, which stabilize the global RNA structure and thus determine the function of the aptamer domain.