Zaydīyya (Arabic: زیدیه) is one of the major Shi'a branches, whose followers believe that after Imam Ali (a), Imam al-Hasan (a), Imam al-Husayn (a), Imam al-Sajjad (a) and Zayd b. 'Ali, an Imam is any righteous, knowledgeable, and brave man from the descendants of Lady Fatima (a) who calls people to his own imamate and to whom people pay allegiance for uprising against unjust rulers. This sect appeared in the early 2nd/8th century. Inclination to Mu'tazili theology and being closer to Sunnis, in comparison to other Shi'a branches, are two characteristics of Zaydiyya. Yemen is currently the country with the largest Zaydi population.
In the past, Zaydis had dynasties in Yemen, Tabaristan, and Morocco. Zaydi Imams ruled in Yemen for about eleven centuries until the establishment of the republic of Yemen in 1962. Since then, Zaydis were mostly in seclusion for two decades, but they have become increasingly active in social and political spheres recently. The revolutionary movement of Ansar Allah is among the influential Zaydi currents of Yemen today.
After the martyrdom of Imam al-Husayn (a), some Alids regarded military revolt against unjust rulers as a condition of imamate. After the demise of Imam al-Sajjad (a), the adherents of this idea supported Zayd b. Ali in his uprising against Hisham b. 'Abd al-Malik and regarded him as their Imam. This marked the formation of Zaydiyya in early 2nd/8th century.