Tacitus was primarily concerned with the concentration of power into the hands of the Roman Emperors. His writings are filled with tales of corruption and tyranny, and display a particular hatred for the emperor Tiberius.
His coverage of the Germanic peoples outside the empire is of mixed value to historians. Tacitus uses what he reports of the German character as a kind of '[noble savage]?' to compare to contemporary Romans and their perceived 'degeneracy'. However, he does supply us with many names for tribes. Indeed, this undependability goes so far that contemporary historians debate whether all these tribes were really Germanic in the sense that they spoke a Germanic language - some of them, like the Batavii, may have been Celts.