MicroRNAs (miRNAs) are small, endogenously expressed, and nonprotein coding RNAs that regulate gene expression via post-transcriptional inhibition and cleavage. To date, several plant miRNAs have been identified via direct cloning, high-throughput sequencing, and bioinformatics analyses. The miRNAs participate in RNA-induced gene silencing complex, and specifically repress the target gene transcripts. Thus, miRNAs regulate the expression of genes playing diverse roles in plants, such as root initiation, leaf morphology, flower development, and response to environmental stimuli. A number of miRNAs have been identified and functionally characterized in eukaryotes. In this review, we discuss the functional roles of miRNAs in plant development as well as stress response to biotic and abiotic environmental factors. Additionally, we present brief information about miRNA detection and discovery techniques.