MicroRNAs (miRNAs) are 20–24 nucleotide noncoding RNAs abundant in plants and animals. The biogenesis of plant miRNAs involves transcription of miRNA genes, processing of primary miRNA transcripts by DICERLIKE proteins into mature miRNAs, and loading of mature miRNAs into ARGONAUTE proteins to form miRNA-induced silencing complex (miRISC). By targeting complementary sequences, miRISC negatively regulates gene expression, thereby coordinating plant development and plant–environment interactions. In this review, we present and discuss recent updates on the mechanisms and regulation of miRNA biogenesis, miRISC assembly and actions as well as the regulatory roles of miRNAs in plant developmental plasticity, abiotic/biotic responses, and symbiotic/parasitic interactions. Finally, we suggest future directions for plant miRNA research.