The Epicurean philosophical system has enjoyed much recent scrutiny, but the question of its philosophical ancestry remains largely neglected. This book traces its origins in the fifth-century BC atomist Democritus, in his fourth-century followers such as Anaxarchus and Pyrrho, and in Epicurus' disagreements with his own Democritean teacher Nausiphanes. The result is not only a fascinating reconstruction of a lost tradition, but also an important contribution to the philosophical interpretation of Epicureanism, bearing especially on its ideal of tranquillity and on the relation of ethics to physics.