Diodorus Siculus was a Greek historian, born at Agyrium in Sicily (now called Agira, in the province of Enna).

Jerome dates Diodorus' floruit to 49 BC (Chronica, s.a. Abraham 1968), which is supported by Diodorus' own statements. The earliest date Diodorus mentions is his visit to Egypt in the 180th Olympiad (between 60 and 56 BC), which was marked by his witnessing an angry mob demand the death of a Roman citizen who had accidentally killed a cat, an animal sacred to the ancient Egyptians (Bibliotheca historia 1.41, 1.83). The latest event Diodorus mentions is Octavian's vengeance on the city of Tauromenium, whose refusal to help him led to Octavian's naval defeat nearby in 36 BC (16.7). Diodorus shows no knowledge that Egypt became a Roman province -- which transpired in 30 BC -- so presumably he published his completed work before that event. Diodorus asserts that he devoted thirty years to the composition of his history, and that he undertook a number of dangerous journeys through Europe and Asia in prosecution of his historical researches; however, modern critics have noted several surprising mistakes that an eye-witness would not be expected to have made.

C.H. Oldfather remarks on the "striking coincidence" that one of only two Greek inscriptions known to him from Agyrium (I.G. XIV, 588) is the tombstone of one "Diodorus, the son of Apollonius".

His history, which he named Bibliotheca historia ("Historical Library"), consisted of forty books, which were divided into three sections. The first six books are geographical in theme, and describe the history and culture of Egypt (book I), of Mesopotamia, India, Scythia, and Arabia (II), of North Africa (III), and of Greece and Europe (IV - VI). In the next section (books VII - XVII), he recounts the history of the World starting with the Trojan War, down to the death of Alexander the Great. The last section (books XVII to the end) concerns the historical events from the successors of Alexander down to either 60 BC or the beginning of Caesar's Gallic War in 45 BC. (The end has been lost, so it unclear whether Diodorus reached the beginning of the Gallic War as he promised at the beginning of his work or, as evidence suggests, old and tired from his labors he stopped short at 60 BC.) He selected the name "Bibliotheca" in acknowledgement that he was assembling a composite work from many sources. The authors he drew from, who have been identified, include: Hecataeus, Ctesius of Cnidus, Ephorus, Theopompus, Hieronymus of Cardia, Duris of Samos, Diyullis, Philistus, Timaeus, Polybius and Posidonius.

This liberal use of earlier historians underlies the harsh opinion of the author of the 1911 Encyclopædia Britannica article on Diodorus:

The faults of Diodorus arise partly from the nature of the undertaking, and the awkward form of annals into which he has thrown the historical portion of his narrative. He shows none of the critical faculties of the historian, merely setting down a number of unconnected details. His narrative contains frequent repetitions and contradictions, is without colouring, and monotonous; and his simple diction, which stands intermediate between pure Attic and the colloquial Greek of his time, enables us to detect in the narrative the undigested fragments of the materials which he employed.

Far more sympathetic is the estimate of C.H. Oldfather, who wrote in the introduction to his translation of Diodorus:

While characteristics such as these exclude Diodorus from a place among the abler historians of the ancient world, there is every reason to believe that he used the best sources and that he reproduced them faithfully. His First Book, which deals almost exclusively with Egypt, is the fullest literary account of the history and customs of that country after Herodotus. Books II-V cover a wide range, and because of their inclusion of much mythological material are of much less value. In the period from 480 to 301 B.C., which he treats in annalistic fashion and in which his main source was the Universal History of Ephorus, his importance varies according as he is the sole continuous source, or again as he is paralleled by superior writers. To the fifty years from 480 to 430 B.C. Thucydides devotes only a little more than thirty chapters; Diodorus covers it more fully (11.37-12.38) and his is the only consecutive literary account for the chronology of the period. ... For the years 362-302 B.C. Diodorus is again the only consecutive literary account, and ... Diodorus offers the only chronological survey of the period of Philip, and supplements the writers mentioned and contemporary sources in many matters. For the period of the Successors to Alexander, 323-302 B.C. (Books XVIII-XX), he is the chief literary authority and his history of this period assumes, therefore, an importance which it does not possess for the other years.

As indicated, Diodorus' immense work has not survived intact: we have the first five books and books 10 through 20. The rest exists only in fragments preserved in Photius and the excerpts of Constantine Porphyrogenitus.

The editio princeps of Diodorus was a Latin translation of the first five books by Poggio Bracciolini at Bologna in 1472. The first printing of the Greek original (at Basel in 1535) contained only books 16-20, and was the work of Vincentius Opsopoeus. It was not until 1559 that all of the surviving books, and surviving fragments of books 21 to the end was published by H. Stephanus at Geneva.