Ernest Gellner's is the best-known modernist explanatory theory of nationalism. This article summarizes its expression and development before considering its strengths and weaknesses. Discussion centres on Gellner's functionalist mode of explanation, the place of nationalism in his philosophy of history, the predictive and retrodictive nature of his theory, and the merits of his typology of nationalism. The apolitical character of his writings is emphasized: in particular, though Gellner established the connections between nationalism and egalitarianism in modern societies, he did not emphasize the mutually reinforcing relationships between nationalism, egalitarianism and democratization; moreover, his contempt for nationalist doctrines is not something liberals, socialists and conservatives need share.