MicroRNAs (miRs) comprise a class of tiny (∼ 19-24 nucleotide), noncoding RNAs that regulate gene expression posttranscriptionally. Since the discovery of the founding members lin-4 and let-7 as key regulators in the developing nematode, miRs have been found throughout the eukaryotic kingdom. Functions for miRs are wide-ranging and encompass embryogenesis, stem cell biology, tissue differentiation, and human diseases including cancers. In this chapter, we begin by acquainting our readers with miRs and introducing them to their biogenesis. Then, we focus on the roles of miRs in stem cells during tissue development and homeostasis. We use mammalian skin as our main paradigm, but we also consider miR functions in several different types of adult stem cells. We conclude by discussing future challenges that will lead to a comprehensive understanding of miR functions in stem cells and their lineages.