MicroRNAs as the endogenous mediators of RNA interference have experienced an unprecedented career in recent years, highlighting their pathogenic, diagnostic and potential therapeutic relevance. Beside tissue microRNAs, they are also found in body fluids, most notably in blood. Significant differences of circulating microRNA levels have been found in various diseases, making them candidates for minimally invasive markers of disease, for example tumor malignancy. The book focuses on the potential diagnostic applicability of circulating microRNAs in various diseases and their potential biological significance.