MicroRNAs (miRNAs) are a recently discovered class of small RNA molecules that regulate gene expression at the post-transcriptional level.MiRNAs maintain the appropriate balance important process in tumor development including proliferation, differentiation,apoptosis, senescence, angiogenesis and metastasis. MiRNA deregulationmayhave profoundconsequences in the cell functionality, especially because individual miRNAs can modify the expression of multiples proteins. MiRNAs exhibit differential expression levels in cancer and have demonstrated capability to affect cellular transformation, acting either as oncogenes or tumour suppressors.In this chapter we discuss the role as oncogenes or tumor suppressor of miRNA in cancer from a traditional point of view considering them astumor suppressors when they impair tumor progression or as oncogenes when they promote it. However, since the effects of the miRNAs are intrinsically pleiotropic, this classification should be considered flexible. Interestingly, miRNA expression profiles may become useful biomarkers for cancer diagnostics and prediction. In addition, miRNA therapy could be a powerful tool for cancer prevention and therapeutics.Although the microRNA era started only a few years ago, it has brought great promise for clinical applications of many human pathologies, including cancer.