Abstract MicroRNAs (miRNAs) are small, endogenous RNAs that regulate gene expression in plants and animals. In plants, these 21-nucleotide RNAs are processed from stem-loop regions of long primary transcripts by a Dicer-like enzyme and are loaded into silencing complexes, where they generally direct cleavage of complementary mRNAs. Although plant miRNAs have some conserved functions extending beyond development, the importance of miRNA-directed gene regulation during plant development is now particularly clear. Identified in plants less than four years ago, miRNAs are already known to play numerous crucial roles at each major stage of development—typically at the cores of gene regulatory networks, targeting genes that are themselves regulators, such as those encoding transcription factors and F-box proteins.