Perun was the Slavic god of thunder, of storms, of war, of oath-binding — the supreme god of the pre-Christian Kievan pantheon and (by the analogy of his name and function) the principal sky-god of the Slavic peoples generally. His name is etymologically secure: from the Indo-European root *per-/*perkw-, "to strike," the same root that gave the Vedic Parjanya, the Lithuanian Perkūnas (treated in the next book), and (with prefix-loss) Greek keraunos ("thunderbolt"). His weapon was a great axe or hammer that returned to his hand when thrown — a striking parallel to Thor's Mjölnir, suggesting either deep Indo-European inheritance or borrowing from the Germanic neighbors. His sacred tree was the oak (whose tall stature and frequent lightning-strikes connected it with thunder-gods across Indo-European traditions); his cult-images stood in oak-groves; his priests wore beards.

The Russian Primary Chronicle records that Vladimir the Great, before his Christianization in 988, set up a great cult-image of Perun at Kiev with a head of silver and a moustache of gold; at his conversion, the same Vladimir had the image torn down and dragged through the city by oxen, beaten by twelve men, and thrown into the Dnieper river — a vivid attested moment of pagan-Slavic destruction at the conversion. The Chronicle's narrative has been confirmed in its essentials by archaeological finds at Kiev. The cult of Perun thereafter survived in folk-memory, with the prophet Elijah of the Old Testament being identified with him in popular Russian piety (Elijah, like Perun, was a sky-and-fire-bringing figure, and the substitution allowed continued reverence for the storm-god under a Christianizing name).