microRNA (miRNA)-mediated RNA interference has been identified as a novel mechanism that regulates protein expression at the translational level. Recent publications have provided compelling evidence that a range of miRNAs are involved in the regulation of immunity, including the development and differentiation of B and T cells, proliferation of monocytes and neutrophils, antibody switching and the release of inflammatory mediators. In this review, we examine what is presently known of the function and mechanism of action of these miRNAs in the regulation of the innate and acquired immune response.