MicroRNAs (miRNAs) are a group of endogenous, small noncoding RNAs of approximately 22 nucleotides in lengths. As a new class of signaling modulators, miRNAs have attracted great attention for their unique features, including multitarget regulation, tissue specificity, and evolutionary conservation. These small endogenous RNAs are able to interact with many important genes and play critical roles in a wide range of biological processes, including cell proliferation and differentiation. Strikingly, miRNAs are frequently dysregulated in human cancers. A number of studies have shown that miRNAs are involved in cancer pathogenesis by regulating oncogenes or tumor suppressor genes. Here, we review recent studies of miRNAs in cancer development and discuss their potential applications in cancer therapeutics.