MicroRNAs (miRNAs) are approximately 22 nucleotide endogenous RNA molecules which exert their functions by base pairing with messenger RNAs (mRNAs), thereby regulating protein-coding gene expression. In eukaryotic cells, miRNAs play important roles in regulating biological processes such as proliferation, differentiation, apoptosis, and stem cell self-renewal. miRNAs are encoded by the genome, and more than 1,000 human miRNAs have been identified so far. miRNAs are predicted to target ~60% of human mRNAs and are expressed in all animal cells. Unique expression domains, targets, and gain- and loss-of-function phenotypes of particular miRNAs have important implications for directed to control differentiation of stem cell populations. Many cancers show variations in miRNA levels, and more specifically an overall downregulation, when compared to their normal counterparts. Therefore, miRNAs may be used as potential therapeutic agents to correct aberrant transcript levels found in the signaling pathways of cancer. This review examines the most recent acquisition on the role of miRNAs in regulating the cell cycle, with particular emphasis on their effects on cell proliferation and differentiation. The second part explores specifically the role of these factors in the physiological regulation of embryonic stem cells, of cellular reprogramming and their involvement in the activation of stem cells in adult tissues. In the third part, the article discusses some issues that relate to the role of miRNAs in the development of neoplastic diseases, focusing on aspects of the genetic and transcriptional alterations that determine the beginning and the development of tumor process, with emphasis on, looking to emphasize their involvement in the activation of adult cancer stem cells.