Epicurus’ Letter to Herodotus was written in 307/6 BC and is a summary of the first thirteen books of his major work on physics called On Nature, of which only fragments survive. The letter is our most detailed and longest extant Greek account of ancient atomic physics, and summarises the following topics: the first principles of the universe, along with its constituents and the fact that everything consists of ‘atoms and void’; the infinity of the universe; how we perceive the world; the properties and parts of compounds and atoms; the motion and speed of bodies in the universe; the physical nature of the soul; the nature of time; the existence of other worlds than ours; the development of language; the motion of heavenly bodies like the sun, moon, and stars; and the nature of the divine. All of this is in service of the search for happiness, which Epicurus believed was partly achieved through the study of nature, through which we can elimate fears of the unknown, the afterlife, and punishment by the gods. The last complete commentary on this text in English was in 1926, by Cyril Bailey. Other scholarship has of course been published since then, but for the most part only considers parts of the letter, often taken out of context, with scholars sometimes resorting to explaining away problems in Epicurus by pointing to Lucretius, the 1st century BC poet from whom we have the most complete record of Epicurus’ philosophy. Although acceptable when trying to reconstruct a full Epicurean theory, it sidesteps the question of how a contemporary reader would have approached and dealt with this didactic text. My aim is to show that reconstructing much of Epicurus’ thinking on physics is possible with the letter alone.