MicroRNAs (miRNAs) are important regulators of gene expression in the immune system. In a few short years, their mechanism of action has been described in various cell lineages within the immune system, targets have been defined and their unique contributions to immune cell function have been examined. Certain miRNAs serve in important negative feedback loops in the immune system, whereas others serve to amplify the response of the immune system by repressing inhibitors of the response. Here, we review some of the better understood mechanisms as well as some emerging concepts of miRNA function. Future work will likely involve defining the function of specific miRNAs in specific immune cell lineages and to utilize them in the design of therapeutic strategies for diseases involving the immune system.