Mitochondria, the powerhouse of the cell, known for producing energy through oxidative phosphorylation and the Krebs cycle, continue gaining notoriety for roles beyond bioenergetics. Recently in Nature, Weinberg et al. (2019) reported that mitochondrial complex III is indispensable for suppressive function of regulatory T cells, thus highlighting the importance of mitochondria in immune tolerance. © 2019 Elsevier Inc. Mitochondria, the powerhouse of the cell, known for producing energy through oxidative phosphorylation and the Krebs cycle, continue gaining notoriety for roles beyond bioenergetics. Recently in Nature, Weinberg et al. reported that mitochondrial complex III is indispensable for suppressive function of regulatory T cells, thus highlighting the importance of mitochondria in immune tolerance. © 2019 Elsevier Inc.