MicroRNAs (miRNAs) are a category of non-coding RNAs which play a crucial role in the regulation of gene expression. A vast range of miRNAs are transcribed from DNA sequences in primary miRNAs, which are then processed into precursor miRNAs and then into mature miRNAs. The interaction of miRNAs with their target genes is dynamic and depends on a variety of factors including the abundance of miRNAs and target mRNAs, the affinity of miRNA-mRNA interactions, and miRNA subcellular location. They can be secreted into extracellular fluids and transferred to target cells by vesicles like exosomes or through a bind to proteins such as argonautes. Furthermore, miRNAs regulate the activities of host cell and also secrete and transport those host cells to recipient cells. This book provides significant information to help develop a good understanding of microRNAs. The readers would gain knowledge that would broaden their perspective in this area of study.