Within the past few years, studies on microRNA (miRNA) and cancer have burst onto the scene. Profiling of the miRNome (global miRNA expression levels) has become prevalent, and abundant miRNome data are currently available for various cancers. The pattern of miRNA expression can be correlated with cancer type, stage, and other clinical variables, so miRNA profiling can be used as a tool for cancer diagnosis and prognosis. miRNA expression analyses also suggest oncogenic (or tumor-suppressive) roles of miRNAs. miRNAs play roles in almost all aspects of cancer biology, such as proliferation, apoptosis, invasion/metastasis, and angiogenesis. Given that many miRNAs are deregulated in cancers but have not yet been further studied, it is expected that more miRNAs will emerge as players in the etiology and progression of cancer. Here we also discuss miRNAs as a tool for cancer therapy. Copyright © 2009 by Annual Reviews. All rights reserved.