MicroRNAs (miRNAs) are a newly identified class of small regulatory non-coding endogenous RNAs that are ubiquitous from animals to plants and have pivotal functions in nearly all biological and metabolic processes. Increasing evidence shows that miRNAs are also new players regulating many protein-coding genes and specific pathways during the cell cycle. This review focuses on the functions of miRNAs in the cell cycle of differentiated cells and embryonic stem (ES) cells. Aberrant expression of these cell-cycle-related miRNAs may result in carcinogenesis, revealing the potential of miRNAs as therapeutic targets for clinical purposes. J. Cell. Physiol. 225: 296–301, 2010. © 2010 Wiley-Liss, Inc.