Many critics and commentators hold that Heidegger had next to nothing to say about human sociality. In this book, Nicolai Knudsen rectifies this popular misconception. Drawing on his influential philosophy of mind, his philosophy of action and his conception of being-with, Knudsen argues that the central idea of Heidegger's social ontology is that we can only understand others, do things with others, and form lasting groups with others if we pre-reflectively correlate their behaviour with our own projects and the world that lies between us. Knudsen then uses this framework to formulate Heideggerian contributions to current debates on social cognition, collective intentionality, and social normativity. He also reinterprets Heidegger's famous concept of authenticity in the light of his social ontological commitments, and shows how Heidegger's affiliation with National Socialism betrays his own best insights into the fundamental structure of social life.