Once regarded as a biological anomaly, microRNAs (miRNAs) have since been recognized as a prevalent RNA species that regulates a wide array of biological processes, from fat storage and insulin secretion, to apoptosis and cell growth. Recent studies show that miRNAs are expressed at precise times and locations in embryonic development. Moreover, disruption of miRNA processing triggers widespread developmental defects. These findings bolster the idea that miRNAs also regulate multiple aspects of embryonic development. This primer focuses on the emerging roles of miRNAs in development. The basics of miRNA biogenesis and miRNA and mRNA target identification are covered, with an emphasis on miRNA function in development. The primer also features a dialog about current topics in the field. Developmental Dynamics 235:846–853, 2006. © 2006 Wiley-Liss, Inc.