The De Rerum Natura of Lucretius (?97-55 BC) is at first sight something of an oddity: a scientific treatise dealing with atomic physics, human biology and the nature of the cosmos, it is at the same time a poem of great power and intensity, one of the most important and influential literary works of its era. This book seeks to resolve the apparent contradiction by locating Lucretius’ poem in the context of a very ancient tradition of didactic (or ‘teaching’) epic. It explores some of the ways in which Lucretius, in this attempt to convince the reader of the truth of his philosophical system, makes the traditional features of epic poetry work for him. And it discusses the poet’s subtle interweaving of technical exposition with ethical precept, arguing that the poem offers the reader not just a scientific account of the workings of nature, but also a guide to happiness.