MicroRNA (miRNA) refers to a small single-stranded non-coding RNA molecule. It can be found in animals, plants and some viruses. miRNAs function in post-transcriptional regulation of gene expression and RNA silencing. They work through base-pairing along with complementary sequences in mRNA molecules. Several mammalian cell types contain a high concentration of miRNAs as circulating in extracellular fluid, which are released into body fluids such as blood and cerebrospinal fluid, and could be used as biomarkers in a variety of disorders. MiRNAs play a role in gene regulation and can also produce histone modifications and DNA methylation at promoter sites, which can affect the target gene expression. MiRNAs are important in modulating complex enzymatic cascades, such as the hemostatic blood coagulation system. The topics included in this book on microRNAs are of utmost significance and bound to provide incredible insights to the readers. Researchers and students engaged in the study of this molecule will be assisted by it.