In the past two decades, small silencing RNAs (ssRNAs or sRNAs) have emerged as one of the most exciting and revolutionary discoveries in molecular biology. ssRNAs are noncoding molecules used by eukaryotes in transcriptional and posttranscriptional gene silencing, endogenous mechanisms for gene regulation and defense against invasive nucleic acids. MicroRNAs (miRNAs), a type of ssRNAs, play a pivotal role in endogenous regulation of gene expression via cleavage or translational repression of complementary "target" mRNA molecules. Plant miRNAs were first described in Arabidopsis in 2002 and now have been identified in more than 120 plant species (including crops and model species); thus, plant miRNA research is one of the fastest growing fields in molecular biology. The first families of miRNAs identified were known to mainly target transcription factors and were involved in growth and development regulation. The initial tools for miRNA identification were often time consuming, expensive, and difficult. In the last few years, various "omics" approaches have been developed to study miRNAs, allowing the spread of miRNA studies to a larger number of plants and the discovery of novel miRNAs and their functions. Notably, various miRNAs are now known to play a role in biotic and abiotic stress, which has led researchers to consider them as a promising tool to develop stress-resistant crops. In this chapter, we review the role miRNAs play in the way plants react to biotic and abiotic stress. We highlight the new methodologies being used to discover miRNAs and to understand their functions in plant responses to environmental stimuli. We also discuss and emphasize the possibilities of generating crop plants with increased stress tolerance using microRNA-mediated technologies.