Diodorus was born around 80 BCE in Agyrium, a Greek city in Sicily near modern Agira. Nothing certain is known about his family, education or early career. His own work supplies the few secure dates of his life. He says that he visited Egypt during 60-57 BCE, where he met priests and foreign ambassadors and gathered information about Egyptian antiquities. He also says that he spent several years in Rome. He claims more generally to have travelled through Asia and Europe, but the Egyptian journey and residence in Rome are the only travels confirmed by specific internal evidence.

He wrote in Greek for a Greek-speaking readership while Rome was conquering and reorganising the Mediterranean. His lifetime spans the collapse of the Roman Republic and the establishment of the Augustan principate. His history ends before the civil wars between Caesar and Pompey reach their final crisis, but his references to Caesar, Roman power and the transformation of the state show that he wrote under the pressure of contemporary events.

He says that his history took thirty years to compose. Jerome's version of Eusebius' Chronicle places Diodorus' fame in 49 BCE, perhaps because an unfinished or pirated portion of his work appeared then. Diodorus himself complains that certain books were stolen and published before his final revisions. The completed Bibliotheca appears to have reached the public around 30 BCE. No ancient source gives his death date, and the traditional c. 20 BCE is approximate.

His one secure work is the Bibliotheca Historica, also called the Historical Library or Library of History. The Greek word bibliotheca means library, and Diodorus uses the title to advertise the work's purpose: not a fresh political history built from autopsy alone, but a universal storehouse assembled from many earlier historians. He selected, arranged and adapted earlier accounts into a continuous narrative from the origins of the world to his own age.

The forty books fall into three broad divisions. Books One through Six treat the mythical and earliest history of the peoples of the known world. Diodorus outlines his plan at the beginning: the first three books contain the antiquities of the barbarians or non-Greeks, and the next three contain the antiquities of the Greeks. The narrative is geographical as well as chronological, moving from Egypt to the Near East, India, Scythia, Arabia, Africa and then the Greek lands.

Books Seven through Seventeen continue from the Trojan War to the death of Alexander the Great. Books Eighteen through Forty carry the history from the successors of Alexander through the Hellenistic kingdoms and the rise of Rome to approximately 60 BCE. Diodorus promises in the preface to bring the narrative to the beginning of Caesar's Gallic Wars, usually dated to 59 or 58 BCE. The surviving fragments do not show clearly whether he fulfilled that plan or stopped around 60 BCE.

Fifteen of the forty books survive complete: Books One through Five and Books Eleven through Twenty. The missing portions survive in fragments, often substantial, from papyri, quotations, Photius and the Byzantine Excerpta commissioned under Constantine VII Porphyrogenitus. The loss of Books Six through Ten removes most of the transition from mythical Greece to the historical period. The loss of Books Twenty-one through Forty removes the central narrative of the Hellenistic age and the late Roman Republic.

Book One describes Egypt. Diodorus presents the Nile, the flooding of the river, Egyptian religious practices, animal cults, the power of the priests, the lives of the pharaohs and the antiquity claimed for Egyptian civilisation. He gives a rationalising account of Egyptian gods and heroes, treating many of them as early kings, inventors or culture bringers. His Egyptian priests claim an antiquity of more than ten thousand years. The book preserves traditions from Hecataeus of Abdera and other Hellenistic writers, while also reflecting Diodorus' own visit to Egypt.

Book Two moves through the Near East. It describes Assyria, Ninus and Semiramis, the Medes and Persians, the customs of India, the Brahmans, the Scythians, the Amazons and the peoples of Arabia. Diodorus preserves fantastic reports alongside geographical and ethnographic information, including traditions about gold, elephants, distant rivers and strange forms of government. The book is one of the most important witnesses to Hellenistic ideas about the East.

Book Three covers Africa. It describes Ethiopia, Libya, the Atlas mountains, the gold mines, the gods and heroes of North Africa, the Libyan Amazons and the peoples of the interior. Diodorus incorporates traditions from Agatharchides of Cnidus and other Hellenistic writers, preserving accounts of mining, caravan routes and distant lands that would otherwise survive only in fragments. His Africa joins geography, myth, marvel and political history.

Books Four and Five describe the Greek world. Book Four presents Greek myths and heroes, including the origins of the gods, Heracles, Theseus, the Argonauts, the house of Oedipus, the Trojan cycle and the legendary history of early Greece. Diodorus often turns myth into rationalised history, explaining wonders as misunderstandings or exaggerations. He preserves moral lessons about piety, justice and punishment while presenting gods and heroes as culture givers, kings or extraordinary mortals.

Book Five is geographical. It covers Crete, the Aegean islands, mainland Greece, Europe and the peoples of the north. Diodorus describes mountains, rivers, harbours, cities, local myths and the origins of place names. He preserves stories about Cadmus, Cecrops, Pelasgus, the autochthonous inhabitants of Attica, the earliest inhabitants of Arcadia and the transmission of agriculture, writing and law. The book gives the Greek mythical world a physical map.

Book Six, now fragmentary, continued the early Greek narrative toward the Trojan War. The surviving fragments preserve portions of mythic history and the transitions between heroic generations. Books Seven through Ten, also fragmentary, carried the story from the returns of the Greek heroes through the migrations, the founding of cities and the archaic age. These books contained the history of the early Greek world before 480 BCE and are among the most regrettable losses in the corpus.

Book Eleven opens the complete historical section with 480 BCE, the year of Salamis. Diodorus narrates Xerxes' invasion, the battles of Salamis, Plataea and Mycale, the growth of the Athenian empire and the wars of the early 5th century. He follows Greek, Persian and Sicilian events together, attempting a synchronistic universal history. His account of the Persian Wars depends heavily on earlier historians and preserves details that supplement Herodotus and Thucydides.

Book Twelve covers 455-416 BCE and includes the transfer of the Delian League treasury to Athens, the wars between Athens and Sparta, Sicilian history and the growth of Athenian power. Diodorus follows an annalistic format, dating by Athenian archons and Roman consuls. His synchronistic method places events in Greece, Sicily, Italy and Persia within the same chronological framework, even when a connected story would be easier to follow.

Book Thirteen narrates 415-405 BCE. It covers the Athenian expedition to Sicily, the siege of Syracuse, the destruction of the Athenian fleet, Alcibiades' career, Cyrus' revolt against Artaxerxes and the final years of the Peloponnesian War. His account of the Sicilian expedition uses Philistus and other sources, while the eastern sections preserve material of major historical value. Errors appear, but the scale of detail is enormous.

Book Fourteen continues from the Athenian defeat to 387 or 386 BCE. It includes the rise of Dionysius I of Syracuse, Cyrus' expedition and the battle of Cunaxa, the trial of the generals, the career of Agesilaus, the Corinthian War and the King's Peace. Diodorus preserves the history of Sicily, Persia, mainland Greece and Italy within a single annalistic structure.

Book Fifteen covers 386-361 BCE. It narrates the Theban revival, the campaigns of Epaminondas and Pelopidas, the Sacred War, the rise of Mausolus, Dionysius of Syracuse and the struggles of the Greek cities. The book preserves important information about fourth-century politics and is particularly valuable because it survives where other continuous accounts are absent or incomplete.

Book Sixteen covers 360-336 BCE and focuses on the rise of Philip II of Macedon. Diodorus describes Philip's wars, diplomacy, marriages, reform of the Macedonian army, the Sacred War, the battle of Chaeronea and the establishment of Macedonian supremacy. The book also narrates the Third Sacred War, the Phocians, the Thebans and the affairs of Sicily and Italy. It is the most extensive surviving continuous narrative of Philip's reign, even though Diodorus' source use is uneven.

Book Seventeen is dominated by Alexander the Great. It covers 336-323 BCE from Alexander's accession through the conquest of Persia to his death in Babylon. Diodorus preserves the major battles, sieges, policies and personal conflicts of Alexander's reign. His account is less analytical than those of Arrian and Plutarch but contains valuable traditions from historians of Alexander that would otherwise be lost. He also includes material on the Persian court and the administrative consequences of conquest.

Books Eighteen through Twenty describe the wars of Alexander's successors. Book Eighteen begins with Alexander's death and the division of the empire. It covers the Lamian War, the destruction of Thebes, the struggles of the generals and the rise of Antipater. Book Nineteen continues the wars among Ptolemy, Seleucus, Antigonus, Demetrius and Cassander, including the siege of Rhodes and the early formation of the Hellenistic kingdoms. Book Twenty reaches 302 or 301 BCE and sets the stage for the battle of Ipsus.

Books Twenty-one through Forty survive only in fragments. They covered the Hellenistic monarchies, the wars of Pyrrhus, the Ptolemies and Seleucids, Macedon, the Achaean and Aetolian leagues, Carthage, Sicily, the Punic Wars, the eastern campaigns of Rome and the civil wars of the late Republic. Book Thirty-six preserves material on the Sicilian slave revolts. The final books reached the age of Caesar and the wars that transformed Rome into an empire.

Diodorus names many sources directly, but the exact relationship between the Bibliotheca and those sources remains debated. For the early books he used Hecataeus of Abdera, Ctesias, Agatharchides and Hellenistic writers on Egypt and the East. For Greek history he relied heavily on Ephorus, Theopompus, Timaeus and other fourth-century historians. Hieronymus of Cardia probably supplied much of the narrative of the successors. Polybius, Duris, Diyllus, Philistus and Posidonius contributed to later books. His quality varies because his sources vary.

His method was usually to follow one principal source at a time. When that source ended, he changed to another. He also inserted speeches, moral judgements, geographical notes and material from other readings into the narrative. This creates unevenness. A book may be detailed, rhetorical and historically valuable in one passage and careless, compressed or chronologically confused in another.

His chronology is particularly difficult. He dates by Athenian archons and Roman consuls, attempting to synchronise events that began in different months of the year. A story that lasted several years may be placed under the archon year in which its most important event occurred. Events in different regions are crowded into the same annual entry, and transitions between sources can produce chronological dislocations. His dates therefore require careful comparison with other evidence.

His errors are real. He can confuse Persian satraps, duplicate battles, misplace events or compress several years into one. Yet the common description of him as a merely uncritical compiler is too simple. The Bibliotheca displays consistent themes: history should benefit its readers, rulers who act unjustly provoke rebellion, greed is the root of many evils, the gods punish impiety, and moderate rule lasts longer than harsh domination. His prologues, judgements and moral digressions reveal an author with a worldview, even when the details come from earlier writers.

His treatment of myth deserves separate attention. In Books One through Six he does not simply repeat Homer and Hesiod. He turns myths into ethnographic history, describing gods as ancient kings and culture bringers. Orpheus, Dionysus, Demeter, Heracles and other divine figures become civilisers, lawgivers and conquerors who travel through the world introducing agriculture, writing, music and justice. His method preserves rationalising, euhemeristic and philosophical traditions that show how Hellenistic Greeks interpreted their myths.

His Sicilian perspective is visible throughout the work. He gives Sicily and southern Italy more space than some universal historians would have done. He describes the tyrants of Syracuse, the wars of the Greek cities, the indigenous peoples of the island and the Roman conquest of Sicily. His hometown Agyrium lies in the interior, and his history gives the Greek West a voice within the larger Mediterranean narrative.

His Roman material is also substantial. Diodorus includes Roman history from the earliest kings through the Republic, the Punic Wars and the civil wars. His account preserves traditions that supplement Livy and Dionysius of Halicarnassus. He shows interest in Roman institutions, colonies and wars, but his viewpoint remains Greek. Rome is the rising power within a Mediterranean of Greek cities, Hellenistic monarchies, Carthaginians, Italians and indigenous peoples.

The ancient reception of the Bibliotheca began with Pliny the Elder, who praises Diodorus' title and mentions his work in the Natural History. Photius read and summarised the forty books in the 9th century. The Byzantine emperor Constantine VII commissioned excerpts from many historians, including Diodorus, and the Excerpta preserve crucial fragments. Eusebius used Diodorus in his Chronicle and Praeparatio Evangelica. The Suda repeats ancient biographical information and excerpts from the work.

The medieval and Renaissance transmission was difficult. The complete books circulated in a handful of manuscripts, while the fragments depended on Byzantine collections. Scholars reconstructed the lost books by comparing excerpts, quotations and epitomes. Poggio Bracciolini's Latin translation helped introduce Diodorus to Renaissance Europe. Wesseling's 1746 edition, Dindorf's text, Vogel's Teubner edition and C.H. Oldfather's twelve-volume Loeb translation established the modern form of the corpus.

Modern scholarship has moved away from the simple verdict that Diodorus was an incompetent copyist. His work is now studied as a major example of universal history, as evidence for lost sources, as a witness to Greek identity under Rome and as a narrative shaped by moral and political themes. He is not always accurate, but he is often the only continuous narrative available for events between Herodotus and Polybius. His preservation of the fourth century and the age of Alexander makes him indispensable.

The most accurate description of Diodorus' works is precise. He wrote the Bibliotheca Historica in forty books. Fifteen books survive complete, and the other twenty-five survive in fragments or excerpts. He wrote no secure separate treatise on Egypt, geography, rhetoric, biography or Roman history. Any such material belongs within the Bibliotheca. The work is a compilation in the sense that it uses earlier historians, but it is also an authorial achievement: Diodorus chose the structure, order, sources, digressions and moral framework through which the Mediterranean world received its history.