Regulatory T cells (Treg) are essential for the establishment and maintenance of immune homeostasis. Due to their proven ability to prevent and even cure autoimmune diseases and their important role in preventing rejection of transplanted organs in preclinical animal models, Treg have entered clinical trials. In contrast, T follicular helper (Tfh) cells provide classic "help" to B cells and promote autoimmunity. microRNAs are posttranscriptional regulators of gene expression. Using in vivo gene ablation we have demonstrated that Treg and Tfh depend on miRNA function in vivo. Specifically, we showed the importance of the miRNA cluster miR-17-92 for Treg and Tfh function. It is intriguing that miR-17-92 regulates both, promoters and inhibitors of immune responses, respectively. We propose to investigate cellular contexts in which miR-17-92 is important and to define molecular mechanisms by which miR-17-92 regulates T cell function. I expect that by studying miRNAs and their target genes we will significantly advance our understanding of immune regulation.