Mitochondria are important signalling organelles, and they dictate immunological fate. From T cells to macrophages, mitochondria form the nexus of the various metabolic pathways that define each immune cell subset. In this central position, mitochondria help to control the various metabolic decision points that determine immune cell function. In this Review, we discuss how mitochondrial metabolism varies across different immune cell subsets, how metabolic signalling dictates cell fate and how this signalling could potentially be targeted therapeutically.