The puranas speak of the ten avataars of Vishnu. These incarnations detail the divine help given by Vishnu during various stages of human evolution, by appearing on earth in different forms. These avatars are said to demonstrate that divinity re-establishes Dharma or righteousness and destroys injustice from time to time, by appearing on earth in various incarnations.

The ten avataras are Matsyavatara (fish), Koorma (tortoise), Varaaha (boar), Narasimha (the man lion), Vaamana (the dwarf), Parasurama (the angry man), Rama (the perfect human), Balarama and Krishna (the divine statesman). The 10th avatar which is yet to appear is Kalki.

Matsyavatara, or the form of the fish was taken up by Vishnu during a deluge that submerged the earth. Vishnu commanded a rishi to gather together samples of all species and wait in a boat. The gigantic golden fish then dragged the boat through the deluge and then enabled Bhrama to start the act of creation all over again. Yet another legend has it that a demon once stole the four Vedas and hid them under the sea. Vishnu assumed the Matsya form and retrieved them and then restored them to their original fragrance. This legend is held at the Parimalaranganathar temple at Mayiladuturai, near Thanjavur (Tamilnadu).

The Koorma Avatara where Vishnu took the form of a tortoise, is described in the legend of the celestial nectar Amrita.

Vishnu took the form of a wild boar - the Varaaha Avatara, dived into the ocean, and saved Bhoomi Devi who sank into the bottom of the ocean, with his massive snout. Vishnu as Varahamurthy is enshrined at Tiruvidandai near Chennai, and at the Kanchipuram Kamakshiamman temple.

In his Narasimhaavataaram, Vishnu destroyed the demon king Hiranyakashipu and demonstrated his omnipresence in a powerful way.

Vishnu took the form of Vamana the midget to destroy the demon Bali. He visited Bali during a sacrifice where the latter was arrogantly distributing gifts of the seeker's choice, to show his power of wealth. Vamana meekly asked for three feet of land measured by his small feet. All on a sudden he assumed the massive form of Trivikrama, dominating the universe; with his first foot he covered the earth, with the second the heavens. When there was no room for the third, Bali, who never went back on his word offered his head, and Vishnu sent him to the netherworld. Trivikrama is enshrined at Tirukkovilur, Kanchipuram and Sirkazhi in Tamilnadu.

Vishnu then took up the form of Parasurama, to quell the arrogance of the Kshatriya rulers who harmed the sages and unprotected mortals.

The Ramayana speaks of the glory of the Rama Avatar, and the Mahabharata speaks of Balarama and Krishna.

Some schools of thought believe that Krishna was the eighth incarnation of Vishnu and the ninth was Buddha and that Vishnu took up the avataar of Buddha to purify Hinduism of excessive ritualism. He preached detachment, and the middle path consisting of eight fold virtues of right views, right resolve, right speech, right conduct, right livelihood, right effort, right mindfullness and right meditation.

It is believed that at the end of the current epoch, there will be a deluge when Kalki - the tenth and the last avatara of Vishnu, will ride forth on a horse to redeem humankind and re-establish righteousness.