Considerable attention and an enormous amount of resources have been dedicated 
to cancer biomarker discovery and validation. However, there are still a limited 
number of useful biomarkers available for clinical use. An ideal biomarker 
should be easily assayed with minimally invasive medical procedures but possess 
high sensitivity and specificity. Commonly used circulating biomarkers are 
proteins in serum, most of which require labor-intensive analysis hindered by 
low sensitivity in early tumor detection. Since the deregulation of microRNA 
(miRNA) is associated with cancer development and progression, profiling of 
circulating miRNAs has been used in a number of studies to identify novel 
minimally invasive miRNA biomarkers. In this review, we discuss the origin of 
the circulating cell-free miRNAs and their carriers in blood. We summarize the 
clinical use and function of potentially promising miRNA biomarkers in a variety 
of different cancers, along with their downstream target genes in tumor 
initiation and development. Additionally, we analyze some technical challenges 
in applying miRNA biomarkers to clinical practice.