**1. Introduction**

MicroRNA (miRNA) is a kind of endogenous non-coding RNA with a length of 18–25 nucleic acid sequences. It is usually integrated into the RNA-induced silencing complex (RISC) to execute its biological function of degrading mRNA or inhibiting transcription. MicroRNA is highly conservative and has strict temporal and spatial specificity. It plays a key regulatory role in the development of animals and plants, cell proliferation, differentiation and apoptosis, immunity and metabolism, angiogenesis, tumor invasion, and metastasis. Mature miRNA has the disadvantages of small fragment, no poly (A), high similarity among family members, and low expression level. As a result, it is difficult to sensitively and accurately detect miRNA. Therefore, it is very important to establish fast and simple methods with high sensitivity and specificity for miRNA detection [1].

Many miRNA analysis methods, including polymerase chain reaction (PCR), DNA Microarray, and Northern blotting have been developed. Although these traditional strategies are the gold standard methods for miRNA identification, detection, and analysis *in vitro*, it is very difficult for them to achieve accurate and sensitive spatio-temporal information of miRNA in living organisms. Therefore, in this chapter, we summarize the novel strategies for biosensing miRNAs *in vivo*

including hybridization probe assays, strand-displacement reaction (SDR), entropy-driven DNA catalysis (EDC), catalytic hairpin assembly (CHA), hybridization chain reaction (HCR), DNAzyme-mediated assays and CRISPR-mediated assays. These contents involve their principles and methods, including their advantages and shortcomings, in order to provide important help for the further study of related detection technology.
