Ukko was the Finnish god of the sky, weather, harvest, fire, the oak tree and thunder, and the leader of the pantheon.
Ukko's domain and court was in the heavens, where he ruled with his wife Akka, and held the epithets Ylijumala (Supereme God) and Pauanne (Thunder). He also reigned over lesser spirits of his domain, similarly to the sea-god Ahti, the forest-god Tapio and the underworld-god Tuoni.
As the god of the sky, he controlled the weather and all it's phenomena, such as rain, storm and lighting. The god was known to use thunderbolts as weapons, which known as either the Ukon vaaja (bolt of Ukko) or the Ukon nuoli (arrow of Ukko). Other weapons he used was the Ukonvasara (a hammer or axe which could generate lighting) and a flaming sword.
Ukko was worshipped as god of weather and fertility, and received sacrifices in the form sheep-meat which was placed in a birch-bark chest, which in turn was placed at a holy site. The god would eat his share of the sacrifice at night, while the people eat what remains the next day. Warriors also prayed to him, hoping he would drop an Umpiputki, a magic charm which could protect the them during battle.
Ukko has been compared to several other deities, such as Zeus, Jupiter, Thor, Perkūnas, and Perun, who also were gods the sky, thunder and the oak trees, with several of them also using hammer's or axes as weapons.