microRNAs (miRNAs) are small ( approximately 22 nt) noncoding RNAs that have been shown to regulate gene expression post-transcriptionally. They function by pairing with the 3' UTR of target mRNAs and repressing translation or by targeting the mRNA for degradation. miRNAs are involved in diverse aspects of development, maintenance, and disease, and are largely evolutionarily conserved in metazoans. Searching the genomes of organisms from viruses to worms to humans has revealed potentially thousands of miRNA genes. Understanding the patterns of genomic organization between species cannot only help to refine tools to identify new miRNAs, but also provide insight into miRNA biogenesis and function.