Histories (Latin: Historiae) is a Roman historical chronicle by Tacitus. Written c. 100–110, its complete form covered c. 69–96, a period which includes the Year of Four Emperors following the downfall of Nero, as well as the period between the rise of the Flavian dynasty under Vespasian and the death of Domitian. However, the surviving portion of the work only reaches the year 70 and the very beginning of the reign of Vespasian.

Together, the Histories and the Annals amounted to 30 books. Saint Jerome refers to these books explicitly, and about half of them have survived. Although scholars disagree on how to assign the books to each work, traditionally, fourteen are assigned to Histories and sixteen to the Annals. Tacitus' friend Pliny the Younger referred to "your histories" when writing to Tacitus about the earlier work.

By the time Tacitus had completed the Histories, it covered Roman history from AD 69, following Nero's death, to AD 96, the end of Domitian's reign. The Annals deals with the five decades before Nero, from AD 14, the reign of Tiberius, to AD 68, when Nero died.

In one of the first chapters of the Agricola, Tacitus states that he wishes to speak about the years of Domitian, of Nerva, and of Trajan. In the Histories, the project has been modified: in the introduction, Tacitus says that he will deal with the age of Nerva and Trajan at a later time. Instead, he covers the period that started with the civil wars of the Year of Four Emperors and ended with the despotism of the Flavians. Only the first four books and the first 26 chapters of the fifth book have survived, covering the year 69 and the first part of 70. The work is believed to have continued up to Domitian's death on September 18, 96. The fifth book contains—as a prelude to the account of Titus' victory in the First Jewish–Roman War—a short ethnographic survey of the ancient Jews—and is an invaluable record of the educated Romans' attitude towards that people.

Tacitus wrote the Histories 30 years later, not long after Nerva's seizure of power, which bore similarities to the events of the year 69, when four emperors — Galba, Otho, Vitellius, and Vespasian — each took power in quick succession. The mode of their accession showed that because imperial power was based on the support of the legions, an emperor could now be chosen not only at Rome, but anywhere in the empire where sufficient legions were amassed.^{[citation needed]}

Nerva, like Galba, came to the throne by senatorial designation, after the violent death of the previous emperor, Domitian. Like Galba, Nerva had to deal with a revolt of Praetorians and like Galba, he had designated his successor by the traditional expedient of adoption. Galba, described by Tacitus as a feeble old man, had chosen a successor unable, due to his severity, to obtain the faith and the control of the troops. Nerva, instead, had consolidated his power by making a link between the throne and Trajan, who was general of the Upper Rhine legions and popular throughout the army. It is probable that Tacitus was a member of the imperial council in which Trajan was chosen to be adopted.

In the first book of the Historiae, a speech put in the mouth of Galba makes clear Tacitus' ideological and political position. Galba's pure respect for formality and lack of political realism rendered him unable to control events. In contrast, Nerva adopted Trajan, who was able to keep the legions unified, to keep the army out of imperial politics, to stop disorder among the legions, and thus to prevent rival claimants to the throne. Tacitus was sure that only the principatus (the "prince", that is, the monarchical emperor) could maintain peace, the fidelity of the armies, and the cohesion of the empire.

Discussing Augustus Caesar's rise to power, Tacitus says that after the Battle of Actium, the unification of the power in the hands of a prince was necessary to keep the peace. The prince ought not to be a tyrant, like Domitian, nor a fool, like Galba. He should be able to keep the imperium safe, while saving the prestige and the dignity of the Senate. (Seneca addresses the same point). Tacitus considered the rule of the adoptive Emperors the only possible solution to the problems of Empire.