Deoxyribozymes (or DNAzymes) are single-stranded DNA molecules that have the ability to catalyze a chemical reaction. Currently, DNAzymes have to be isolated from random-sequence DNA libraries by a process known as in vitro selection (IVS) because no naturally occurring DNAzyme has been discovered. Several IVS studies have led to the isolation of many RNA-cleaving DNAzymes (RNase DNAzymes), which catalyze the transesterification of a phosphodiester linkage in an RNA substrate, resulting in its cleavage. An RNase DNAzyme and its substrate can be modified with a pair of donor and acceptor fluorophores (or a fluorophore and quencher pair) to create a fluorescence-signaling system (a signaling DNAzyme) where the RNA-cleaving activity of the DNAzyme is reported through the generation of a fluorescent signal. A signaling DNAzyme can be further coupled with an aptamer (a target-binding nucleic acid sequence) to generate a fluorogenic aptazyme in which the aptamer-target interaction confers an allosteric control of the coupled RNA-cleaving and fluorescence-signaling activity of the DNAzyme. Fluorogenic aptazymes can be exploited as valuable molecular tools for biosensing applications. In this chapter, we provide both a detailed description of methods for isolation of signaling DNAzymes by IVS and general approaches for rational engineering of fluorogenic aptazymes for target detection.