microRNAs (miRNAs) are emerging as important gene expression regulators linked to various biological processes at a posttranscriptional level. miRNAs have been known to play important roles in cell proliferation, cell differentiation, and apoptosis. Recently, accumulate studies indicate that up-regulation of miR-155 has been described in several types of human tumors. miR-155 has been considered to act as an oncogene or a tumor suppressor, depending on tumor system. Silencing oncomiRs or gene therapy approaches could be an effective therapeutic approach against tumor. Here we review the current knowledge on the functional role of miR-155 in the control of various cancers. J. Cell. Physiol. 229: 545–550, 2014. © 2013 Wiley Periodicals, Inc.