Zaydiyya is a branch of the Shi‘a arising out of the abortive revolt of Zayd b. ‘Ali b. al-Husayn in Kufa in 122 AH / 740 CE. During the preparations for the revolt, a part of the Kufan Shi‘a withdrew their support from Zayd in protest against his refusal to condemn unconditionally the early caliphs preceding ‘Ali and backed Zayd’s nephew Ja‘far al-Sadiq as their imam. This schism led to a lasting division of the Shi‘a into a radical and a moderate wing in terms of their religious break with the Sunni Muslim community. The Zaydiyya, as the moderates, did not classify the Sunni Muslims generally as infidels. In political terms, however, they were, in contrast to the radical but quietist Imamiyya, militant, espousing revolt against the illegitimate Sunni rule as a religious duty.
A leading contemporary Islamicist, Professor Wilferd Madelung has made significant contributions to modern scholarship on mediaeval Islamic communities and movements, including Twelver Shi'ism, Zaydism and Ismailism. Educated at the Universities of Cairo and Hamburg, he became Professor of Islamic Studies at the University of Chicago in 1969 and the Laudian Professor of Arabic at the University of Oxford from 1978.
Among his recent publications are Religious Schools and Sects in Mediaeval Islam (London, 1985), Religious Trends in Early Islamic Iran (Albany, NY, 1988), Religious and Ethnic Movements in Mediaeval Islam (Hampshire, 1992), The Succession to Muhammad: A Study of the Early Caliphate (Cambridge, 1997), and with Paul E. Walker An Ismaili Heresiography (Leiden, 1998). He has contributed extensively to The Encyclopaedia of Islam, Encycopaedia Iranica of which he is also a Consulting Editor, and learned journals.