MicroRNAs (miRNAs) are a new class of regulator of gene expression. Initially discovered as regulators of developmental timing in invertebrates, miRNAs have subsequently been implicated in a variety of biologic processes. In recent years, their importance for human disease has become apparent. In particular, there is increasing evidence of their role in cancer, both as oncogenes and tumor suppressors, making them appealing targets for therapy. Furthermore, the variations in the abundance of miRNAs in different tissues and cancers offer a specific 'signature' that can be useful in diagnosis.