Hasan was the first grandchild of Muhammad and according to tradition was named by Muhammad upon a revelation from Allah. The name Hasan means "handsome" in Arabic. Hasan and his brother Hussein were greatly beloved by their grandfather, and numerous hadith testify to this. There are also hadith that exist which claim that Hasan and Husayn will be the leaders of the youth in heaven.
Upon the death of his father at Kufa, a number of people in that town and neighboring regions swore allegiance to Hasan, making him the sucessor to his father. This put him in conflict with Muawiyah, who had been in competition with Ali for the caliphate. Muwaiyah began building an army to face Hasan, but at the same time sent him letters asking him to give up his claim. If Hasan was to abdicate, then Muawiyah would have a legal claim to the caliphate, if however, Muawiyah were to defeat Hasan, he would have absolute power, but his claim would be shaky.
The armies of Hasan and Muawiyah would fight some battles, but eventually Hasan would give up his claim in order to maintain peace among the Muslim people. He would then retire to Medina and was given a pension by Muawiyah. At one point, Muawiyah asked Hasan to help him quell a Kharijite rebellion, but Hasan refused to help, writing that he had given up fighting for the sake of peace.
Some believe that he was poisoned, but this claim is questionable.