miRNAs, ∼20 to 22 nucleotide single-stranded RNA species that play a pivotal role in the regulation of protein-coding genes, are emerging as robust biomarkers for assessing allograft status. Herein, the authors briefly review the biogenesis and function of the miRNAs and provide an overview of the tools to quantify miRNAs in tissues and body fluids. They then review their studies of discovery and validation of alterations in miRNA expression within kidney allografts with or without acute rejection, as well as with or without fibrosis, and summarize published data on miRNA expression patterns in kidney transplant recipients.