Recently, many studies indicated that microRNAs (miRNAs) stably existed in various body fluids, including serum, plasma, saliva, and urine. Such miRNAs that exist in mammalian body fluids are known as circulating miRNAs, and they can transmit signals between cells and regulate intracellular gene expression. Currently, we barely understand the characteristics, sources, secretion, uptake, and functions of newly generated miRNAs. Particularly, it has been shown that certain types of circulating miRNAs can provide effective clinical data, suggesting their roles as novel biomarkers for the early detection of diseases such as cancers, cardiovascular diseases, and diabetes. Therefore, miRNAs have attracted much attention in academia for their promising applications in fundamental research and clinical diagnosis. This review summarizes some of the functional studies that have been conducted as well as the promising applications of circulating miRNAs, and we hope it will benefit other researchers in this field.