Every cellular process is likely to be regulated by microRNAs (miRNAs), and an aberrant expression signature of these small non-coding RNAs is a hallmark of several diseases, including cancer. miRNA expression profiling by microarray techniques has provided a powerful tool to reveal the involvement of these tiny molecules in tumor development and progression, showing that they are differentially expressed in tumors as compared to normal tissues. Moreover, specific miRNA signatures have been associated with histopathological and clinical features, suggesting a potential role of these molecules as prognostic and predictive markers. Focusing then on their biological effects and role in cancer, it has been shown that miRNAs can function as potential oncogenes or oncosuppressor genes, depending on the cellular context and on the target genes they regulate. The possibility to modulate miRNA expression either in vitro and in vivo by developing synthetic pre-miRNA molecules or antisense oligonucleotides have at the same time provided a powerful tool to a deeper comprehension of the molecular mechanisms regulated by these molecules, and suggested the intriguing and promising perspective of their possible use in therapy. Herein we review our current knowledge about the involvement of miRNAs in cancer, and their potential as diagnostic, prognostic and therapeutic tools.