Plant microRNAs (miRNAs) are single-stranded RNA molecules of around 22 nucleotides (nt) in length that are associated with the RNA-induced silencing complex (RISC). They act as post-transcriptional negative regulators of gene expression mainly by guiding cleavage or attenuating the translation of target transcripts. The targets of plant miRNAs often belong to transcription factors families involved in the control of developmental processes and defense responses. In the present paper, we reviewed the recent advances in our understanding of the biogenesis and mechanism of action of plant miRNAs, as well as the regulatory roles in plants.