Nucleic acids are well suited to serve as biosensors for the fast and reliable detection of small organic molecules, such as a number of metabolites or antibiotics, specific nucleic acid sequences, peptides, proteins or metal ions. One of the main advantages of using nucleic acids as biosensors is that they can be modulated to respond allosterically to specific effectors. Thus molecular recognition is transformed directly into a catalytic process with observable results. In particular, catalytic RNA structures, such as the hammerhead and hairpin ribozymes, have been used for biosensor engineering. The review reports on the function mode of nucleic acid biosensors and introduces recent developments and applications in the field.