Aberrant expression and function of microRNAs (miRNAs) in cancer have added a new layer of complexity to the understanding of development and progression of the disease state. It has been demonstrated that miRNAs have a crucial function in oncogenesis by regulating cell proliferation and apoptosis as oncogenes or tumor suppressors. The expression signatures of miRNAs provide exciting opportunities in the diagnosis, prognosis and therapy of cancer. Since miRNAs can function as either oncogenes or tumor suppressor genes in oncogenesis, the potential of using these small RNAs as therapeutic targets opens up new opportunities for cancer therapy by either inhibiting or augmenting their activity.