Short non-coding RNA molecules, microRNAs (miRNAs), post-transcriptionally regulate gene expression in living cells. In recent years, miRNAs have been found in a wide spectrum of mammalian body fluids including blood plasma, saliva, urine, milk, seminal plasma, tears and amniotic fluid as extracellular circulating nuclease-resistant entities. The changes in miRNA spectra observed in certain fluids correlated with various pathological conditions suggesting that extracellular miRNAs can serve as informative biomarkers for certain diseases including cancer. However, the mechanism of generation and a biological role of extracellular miRNAs remain unclear. The current theories regarding extracellular miRNA origin and function suggest that these miRNAs can be either non-specific 'by-products' of cellular activity and cell death or specifically released cell-cell signaling messengers. The goal of this Research Topic is to bring together up-to-date knowledge about the extracellular miRNA and its role in disease diagnostics and, possibly, inter-cellular communication.