MicroRNAs (miRNAs) are a group of small noncoding RNAs that have been identified in a variety of organisms. These small, 18–22-nucleotide (nt) RNAs are transcribed as parts of longer molecules called pri-miRNAs, which are processed in the nucleus into hairpin RNAs of 70–100 nt, called pre-miRNAs, by the double-stranded RNA (dsRNA)-specific ribonuclease Drosha. The function of most miRNAs is not known, but for a few members the participation in essential biological processes for the eukaryotic cell is proven. In this review, we summarize how miRNAs were discovered, their biological functions, and importance in animal development, highlighting their function in proliferation, apoptosis, and cell differentiation. Furthermore, we discuss the deregulation of miRNAs in human diseases and their involvement in tumorigenesis. Birth Defects Research (Part C) 78:180–189, 2006. © 2006 Wiley-Liss, Inc.