The widespread presence of slow-red and fast-white muscles in all vertebrates supports the evolutionary advantage of having two types of motors available for animal movement-a slow economical motor used for most activities, and a fast energetically costly motor used for rapid movements and emergency actions, and actions that require a lot of force. Skeletal muscles are composed of multiple fiber types whose structural and functional properties have only in part been characterized. Further progress in this field is mainly occurring along two directions: Multiomics approaches are providing a global picture of the molecular composition of muscle fibers up to the single fiber and single nucleus level. Signaling studies are identifying many transcription factors and pathways controlling fiber-type specification. These new data should now be integrated into a wider whole-body context by defining the matching between muscle fiber and motor neuron heterogeneity in the neuromuscular system, as well as the relevance of muscle fiber types in systemic homeostatic functions, including metabolism and thermogenesis.