MicroRNAs (miRNAs) are evolutionarily conserved small noncoding RNAs that post-transcriptionally regulate gene expression by targeting specific messenger RNAs (mRNAs) for degradation or translational repression. Recent evidence indicates that miRNA-mediated gene regulation is critical for normal cellular functions, and as much as one-third of human mRNAs may be miRNA targets. Emerging evidence suggests that miRNAs play a key role in the regulation of immunological functions including innate and adaptive immune responses, development and differentiation of immune cells, and the prevention of autoimmunity. Here, we review the mechanisms of miRNA maturation and function, the roles of several miRNAs in immunological functions, and the involvement of miRNAs in disease pathogenesis.