MicroRNAs (miRNAs) act as negative regulators of gene expression in eukaryotes, a discovery that has opened an expanding field of biological research. Plant miRNAs are known to repress gene expression posttranscriptionally, mainly by guiding cleavage but also by attenuating the translation of target transcripts. In addition, it has been shown that plant miRNAs can also act at the transcriptional level by directing the methylation of target chromosomal loci. Genetic and biochemical approaches are quickly broadening our knowledge of the biogenesis and function of plant miRNAs. Computational approaches have uncovered an unexpectedly large number of miRNAs and their targets in plants. The targets of plant miRNAs often belong to families of transcription factors involved in the control of developmental processes. We review the status of research in this dynamic field, summarizing recent advances in our understanding of the biogenesis and mechanism of action of plant miRNAs, as well as in the developmental processes they regulate.