MicroRNAs (miRNAs) are small, noncoding RNAs with important functions in development, cell differentiation, and regulation of cell cycle and apoptosis. MiRNA expression is deregulated in cancer by a variety of mechanisms including amplification, deletion, mutation, and epigenetic silencing. Several studies have now shown that miRNAs are involved in the initiation and progression of cancer. In this review, we briefly describe miRNA biogenesis and discuss how miRNAs can act as oncogenes and tumor suppressors. We also address the role of miRNAs in the diagnosis, prognosis, and treatment of cancer.