- Aptamers are single-stranded DNA or RNA molecules with ligand-binding capabilities. Signaling aptamers refer to aptamers or modified aptamers with recordable signal generation ability. Fluorescence-signaling aptamers, in particular, are valuable molecular tools that can be used to establish important techniques or assays for the interrogation of identities and concentrations of proteins and metabolites. Since standard DNA and RNA aptamers themselves are not inherently fluorescent, modification methods are required for rationally converting non-fluorescent aptamers into fluorescent reporters or for selecting fluorescent aptamers directly from random-sequence DNA libraries by in vitro selection. This article will provide a brief review of various signaling aptamer design strategies as well as a detailed description of methods that can be used to generate, by both rational design and in vitro selection, a special class of signaling aptamers dubbed "structure-switching signaling aptamers." This class of signaling aptamers are designed to function by switching structures from a pre-formed, lowly fluorescent duplex assembly to a ligand-aptamer complex having a higher level of fluorescence.