In ancient Greek philosophy the concept of ataraxia (“absence of disturbance,” “freedom from anxiety,” “tranquility”) rapidly gained importance in the third century BCE during and after the conquests of Alexander the Great († 321 BCE). In this period most of the so-called Hellenistic philosophers—the Epicureans, Stoics, Academic skeptics, and Pyrrhonists—related tranquility to their concept of happiness as a fulfilled life. In this chapter I briefly discuss views of ataraxia in the work of Epicurus, the Stoics Seneca and Marcus Aurelius, and the Academic skeptic Cicero. I then give more space to the role of ataraxia in the Pyrrhonism defended by Sextus Empiricus because it presents us with an attitude to life that may appear particularly helpful to us in the twenty-first century.