For long stretches of Greek history in the classical period, Diodorus Siculus provides the only surviving continuous narrative of events. For this narrative he summarized, however incompetently, the work of earlier and greater historians whose original texts are lost to us. This makes Diodorus an invaluable quarry of the historian and the historiographer alike, but one that can only be used with discretion. We need to get as clear an idea as we can of the way his mind worked, where his account is most likely to be useful, and what sort of distortions to expect when he goes astray. Research into his methods and procedures is thus an urgent necessity. The present study, the fullest ever undertaken for any part of Diodorus, aims to meet the needs of both history and historiography. In the introduction, necessarily substantial, the aims, sources, and methods of Diodorus are examined in detail. The findings of this investigation are then applied in commenting on Book 15, a particularly important book which deals with the crucial years between the King's Peace, concluded in 387/6 BC, and the aftermath of the battle of Mantinea fought in 362 BC.