MicroRNAs (miRNAs) play a key role in diverse regulatory pathways ranging from developmental control, cell differentiation, neural and immune system development. Many miRNAs are located in the human genome and frequently amplified, deleted or rearranged in cancer. Many viruses encode their own miRNAs in order to thrive during the course of their infection cycle. This book discusses the mechanism of miRNA biogenesis, virus encoded miRNAs, and the involvement of miRNAs in specific infectious or non-infectious diseases.