miRNAs are short noncoding RNAs that post-transcriptionally regulate gene expression. miRNAs' ability to inhibit translation of oncogenes and tumor suppressor genes implies they have an involvement in carcinogenesis. Specific miRNA expression signatures have been identified in a variety of human cancers. More recently, occurrence of miRNAs in the blood serum and plasma of humans has been repeatedly observed. miRNA levels in serum and plasma are more stable, reproducible and consistent among individuals of the same species in comparison with other circulating nucleic acids. Circulating miRNAs have been successfully evaluated in a wide range of solid cancers as promising novel noninvasive biomarkers of early disease onset or relapse. Here we describe the origin of circulating miRNAs, principles of their immense stability and proposed functions, and comprehensively summarize studies focusing on their significance in the most frequently studied cancer types in this regard, including breast, colorectal, lung and prostate cancer.