Discovered in Caenorhabditis elegans in 1993, microRNAs (miRNAs) make up a novel class of tiny, ~21–24 nucleotide, non-coding RNA species. Since its identification as a key component of a broadly conserved mechanism that regulates gene expression post-transcriptionally, the miRNA pathway has emerged as one of the most extensively investigated pathways of the past decade. Because of their potential to regulate a large number of protein-encoding genes, miRNAs have been implicated in numerous biological processes, including development, stem cell regulation and human diseases. In this Commentary, we focus on miRNAs and their roles in mammalian stem cells. Following an introduction to the miRNA biogenesis pathway with an emphasis on its regulatory features, we then discuss what is currently known about the roles that miRNAs have in the differentiation and maintenance of embryonic and somatic stem cells of diverse origins. In particular, their roles in stem cell differentiation have been well documented. Insights from these studies provide a paradigm for the function of miRNAs in facilitating cellular transitions during differentiation. By contrast, the roles that miRNAs have in the maintenance of stem cells are less well understood. However, with recent advances, their role as a rheostat that fine-tunes stem cell self-renewal has begun to emerge. Finally, we discuss future studies that will hopefully lead to a comprehensive understanding of the miRNA pathway in stem cells.