The activation of T cells is a tightly regulated process that has evolved to maximize protective immune responses to pathogens while minimizing damage to self‐tissues. A delicate balance of cell‐intrinsic, costimulatory, and transcriptional pathways as well as micro‐environmental cues such as local cytokines controls the magnitude and nature of T‐cell responses in vivo. The discovery of functional small noncoding RNAs called micro‐RNAs (miRNAs) has introduced new mechanisms that contribute to the regulation of protein translation and cellular responses to stimuli. miRNAs are short (approximately 22 bp) RNA species, which bind to mRNAs and suppress translation. Due to their short length and imperfect base pairing requirements, each miRNA has the potential to regulate various pathways through the translational inhibition of multiple mRNAs. The human and mouse genomes each encode hundreds of miRNAs, and studying the function of miRNAs has led to the realization that they play important roles in diverse biological processes from development and cancer to immunity. This review focuses on the function of mir‐155 in T cells and the impact of this miRNA on autoimmunity, tumor immunity, and pathogen‐induced immunity.