The fourth book of Tacitus' Annals has been described as 'the best that Tacitus ever wrote'. It covers the years AD 23-28, beginning at the point where Tacitus noted a significant deterioration in the principate of the emperor Tiberius, and the increasingly malign influence of his 'evil genius' Sejanus. In this new edition the editors present an improved text of Annals IV, explain in detail the difficulties and unusual features of Tacitus' Latin, and discuss the dramatic, structural and literary qualities of the narrative. In the introduction they express radical views on how the Romans wrote history and consider the political, moral and stylistic dimensions of the historiographical tradition. Although intended primarily as a textbook for sixth-forms and undergraduates, the edition contains much which will be of interest to scholars of Latin literature and to Roman historians.