MicroRNAs (miRNAs) are small (20–24 nt), single stranded, regulatory RNA molecules or gene regulators of critical transcriptional or post-transcriptional gene regulation in plants in sequence-specific order that respond to numerous abiotic stresses and animals, non-coding, highly evolutionarily conserved and widely distributed throughout the plant kingdom. MiRNAs are master regulators of plant growth and development, development attenuation under various environmental stresses by stress-responsive miRNAs and plant stress responses and tolerance. Drought, salinity, heat, cold, UV radiation, heavy metal, pathogens, pests and other microbial infections affect survival, growth, development, quality, yield, and production of plants. Stress induced miRNAs down regulate their target miRNAs. This down regulation leads to the accumulation and function of positive regulators, highlighting their roles in stress responses and tolerance. Plant miRNA mediated modifications include overexpression or repression of stress-responsive miRNAs and/or their target complementary or partially complementary gene products, miRNA-resistant target genes, target-mimics and artificial miRNAs. Thus, miRNAs may serve as "genomic gold mines", novel, potent and potential targets in plant genetic manipulations and miRNA-based biotechnology will aid plant improvement and crop-plant tolerance to different environmental stresses. This book reviews our recent understanding of plant microRNAs, biogenesis and functions, computational tools and bioinformatics, regulation of plant growth and development, expression studies, and the role of plant miRNAs in various biotic and abiotic stress-response regulation in plants.