Shi'ite Muslims and Sunni Muslims are the two largest branches of Islam, with the overwhelming majority of Muslims being Sunni. The main difference between the two branches is that Sunni Muslims consider the first four caliphs to be the rightful leaders of the Muslim community after the death of the Prophet Muhammad, while Shi'ite Muslims believe that the first caliph should have been Ali, the prophet's cousin and son-in-law. This difference in belief has led to some historical conflicts between the two branches, although the vast majority of Muslims today coexist peacefully regardless of their branch of Islam.