Tacitus' Histories and Annals together amounted to thirty books; although some scholars disagree about which work to assign some books to, traditionally fourteen are assigned to Histories and sixteen to Annals. Of the 30 books referred to by Jerome about half have survived.

Modern scholars believe that as a Roman senator, Tacitus had access to Acta Senatus—the Roman senate's records—thus providing a solid basis for his work.[3] Although Tacitus refers to part of his work as "my annals", the title of the work Annals used today was not assigned by Tacitus himself, but derives from its year-by-year structure.

Histories (Latin: Historiae) is a Roman historical chronicle by Tacitus. Written c. 100–110, it covers the Year of Four Emperors following the downfall of Nero, which includes the period between the rise of the Flavian Dynasty (69–96) under Vespasian and the death of Domitian.

Publius (or Gaius) Cornelius Tacitus was a senator and a historian of the Roman Empire. The surviving portions of his two major works—the Annals and the Histories—examine the reigns of the Roman Emperors Tiberius, Claudius, Nero, and those who reigned in the Year of the Four Emperors (AD 69). These two works span the history of the Roman Empire from the death of Augustus in AD 14 to the years of the First Jewish–Roman War in AD 70. There are substantial lacunae in the surviving texts, including a gap in the Annals that is four books long.