It has been demonstrated that all the known processes involved in cancer, including apoptosis, proliferation, survival, and metastasis, are regulated by small regulatory noncoding RNAs consisting of approximately 19-25 nucleotides; these are named microRNAs (miRNAs). Both loss and gain of miRNA function contribute to cancer development through the upregulation and silencing, respectively, of different target genes. Experimental evidence indicates that the use of miRNA mimics or anti-microRNAs may represent a powerful therapeutic strategy to interfere with key molecular pathways involved in cancer. This review provides insights about how micro- RNAs act as oncogenes and tumor suppressor genes and how these findings, along with our increasing understanding of miRNA regulation, can be applied to optimize recent miRNA-based technologies and make them suitable for clinical applications.