Mammals have evolved a variety of gene regulatory mechanisms to ensure the proper development of tissues during embryonic organogenesis. Recently, microRNAs (miRNAs) have been shown to regulate protein dosage during mammalian development. miRNAs are tiny RNA molecules that function to regulate diverse cellular processes by inhibiting gene expression posttranscriptionally. Since their discovery in mammals in 2000, much has been learned about the biogenesis, mechanisms of action, and expression of miRNAs. This knowledge combined with the identification of new mRNA targets has provided valuable insights into the functions of these RNA regulatory molecules. It is now clear that miRNAs are involved in modulating a variety of developmental and physiological processes. This review is designed to highlight recent advances in the study of miRNAs with a particular emphasis on their roles in mammalian development and cancer progression. Birth Defects Research (Part C) 78:172–179, 2006. © 2006 Wiley-Liss, Inc.