In Zaydi Islam, the imamate (Arabic: إمامة, romanized: imama) is the supreme political and religious leadership position. In common with other Shi'a sects, it is reserved for Alids—i.e., descendants of Muhammad via Ali ibn Abi Talib and Fatimah. Unlike the Twelver or Isma'ili imamate, however, it was not hereditary and could be claimed by any qualified Alid; nor were its holders ascribed semi-divine attributes of infallibility and miracle-working. Necessary qualifications were a grounding in Islamic jurisprudence and a public call to allegiance, usually in the form of leadership of an uprising against unlawful authority. In practice, this meant there could be several Zaydi imams at the same time, even in the same region, or none at all. This resulted in the unique concept of a 'restricted' imamate for holders of political power who did not meet the scholarly credentials. Moreover, the personal nature of the imamate for a long time prevented the rise of persistent institutions, rendering the Zaydi states established in Yemen and Tabaristan unstable. As a result, the imamate often came to be passed down in hereditary fashion, especially as the Imams of Yemen abandoned Zaydi doctrines for Sunni ones in the 18th century.

Zaydism is a branch of Shi'a Islam established by the followers of Zayd ibn Ali (a great-grandson of Ali ibn Abi Talib, the son-in-law of Muhammad and fourth caliph), who in 740 launched an unsuccessful revolt against the Umayyad Caliphate, in which he died. The revolt failed in large part due to lack of support by the Kufan Shi'a, who were divided over the thorny question of the legitimacy of the first three Rashidun caliphs—i.e., those who ruled before Ali ibn Abi Talib—Abu Bakr, Umar, and Uthman; the more radical Shi'a rejected them as usurpers, while the more moderate ones accepted them as legitimate imams (leaders) since Ali himself had pledged allegiance to them. Zayd's support for the moderate position cost him many followers, who instead followed his nephew, Ja'far al-Sadiq (d. 765). This event separated the Zaydis from the "Imami" variants of Shi'ism (the Twelvers and Isma'ilis) who followed al-Sadiq and his successors.

In Zaydi doctrine, unlike the Imami Shi'a, the imamate is not hereditary, nor is the imam a quasi-divine figure, without sin, possessed of infallibility (ismah), and capable of performing miracles. Likewise, Zaydi doctrine rejects core Imami doctrines like the occultation of imams, esoteric interpretations in Quranic exegesis, or the doctrine of dissimulation of one's beliefs (taqiyya). Apart from the emphasis on the imamate and its restriction to Ali's descendants, the Zaydi position, especially in modern times, is close to mainstream Sunnism, and Zaydism is sometimes considered as the fifth Sunni school (madhhab). While the Imamis considered Sunnis as infidels, their early imams were politically quietist, accepting the rule of the Umayyad and later the Abbasid Caliphate. The Zaydis on the other hand adopted a more moderate religious position closer to Sunni beliefs, but were political radicals, with rebellion against illegitimate Umayyad and Abbasid authority becoming a core Zaydi tenet. As a result, the Zaydis backed a succession of legitimist Alid revolts: the rebellion of Abd Allah ibn Mu'awiya (744–747/8), the uprising of Muhammad al-Nafs al-Zakiyya (762–763), the uprising of al-Husayn ibn Ali al-Abid (786), the Daylam revolt of Yahya ibn Abdallah (792), the revolt of Ibn Tabataba in Iraq (814–815) and of Muhammad ibn al-Qasim in Talaqan (834), and of Yahya ibn Umar in Kufa (864).

These revolts were unsuccessful, proving that a direct confrontation with the caliphal government in the central lands of Islam was doomed to failure. The first successful Zaydi regime was indeed established in Morocco, at the westernmost extremity of the Islamic world, by Idris ibn Abdallah, who fled the suppression of the 786 uprising. During the 9th century, Zaydi missionary efforts gained ground in two other peripheral regions of the Islamic world: the mountains of northern Yemen, and the mountains of Tabaristan, Daylam, and Gilan on the southern shores of the Caspian Sea. A Zaydi emirate was established in Tabaristan in 864, which survived as a regional power until the 12th century. The last remnants of the Caspian Zaydis ruled Lahijan and converted to Twelver Shi'ism in 1526/7. In Yemen, a Zaydi state was established in 897 at Saada, which remains the heartland of Yemeni Zaydism to the present day; a series of Zaydi Imams of Yemen ruled much of the highlands of Upper Yemen in the medieval and early modern period, even though their rule was contested and interrupted by foreign imperial powers, most notably the Ottoman Empire. In the 17th century, the Zaydi Qasimi State evicted the Ottomans and came to rule all of Yemen, but to bolster their own power, the imams gradually moved away from Hadawi Zaydism and adopted doctrines that brought them nearer to Sunni practice and legitimated their hereditary, monarchical rule.

Early Zaydis were divided in two camps, the moderate Batriyya, that was closer to proto-Sunnism and prevailed in the 8th century, and the more radical Jarudiyya, which leaned far more towards early Shi'a doctrines, and eventually became the dominant strain by the 9th century. In common with all Shi'a groups, the Batriyya held that Ali was the rightful successor to Muhammad, being "the most excellent of men" after the Prophet himself. At the same time, unlike the other Shi'a groups, they held that since Ali had consented to obey them, Abu Bakr and Umar were rightful leaders, while Uthman was so for the first half of his reign, prior to his attempts to raise his own, Umayyad clan over all others. From this, the Batriyya accepted that imams did not have to be the most excellent member of the Muslim community, as was the case in the Basran school of Mu'tazilism and in Khariji doctrine. The Jarudiyya on the other hand followed the Imami Shi'a in rejecting the first three caliphs as illegitimate and usurpers of Ali's rightful place, holding that he and his sons Hasan (d. 670) and Husayn (d. 680) had been explicitly designated as successors of Muhammad.

The designation of Ali, Hasan, and Husayn makes them unique among Zaydi imams, although two different traditions exist: one holds that all three were explicitly designated as his successors by Muhammad, while another that they each appointed one another in turn. After the death of Husayn, eligibility for the imamate was left open to any qualified member of the Husaynid and Hasanid lines, although some Zaydi scholars until the 10th century extended eligibility to all Talibids (the descendants of Ali's father, Abu Talib ibn Abd al-Muttalib). A prospective imam had to be of sound body and mind so that he could rule, have an upright personal character, live a life of piety and probity, and have an extensive knowledge of Islamic jurisprudence and religious matters that qualified him as a mujtahid, up to and including the authorship of original works. A list of fourteen attributes was eventually established that qualified a candidate for the imamate.

Qualifications alone were not enough, however; while Zaydi doctrine affirmed the establishment of an imam at all times as an obligation incumbent upon the Muslim community, the imamate could not be passed by contract, election or designation, but had to be claimed by issuing a 'call' or 'summons' (da'wa) which had to be made via public pronouncement to 'rise up' (khuruj)—the terms 'rising' (qiyama) or 'emergence' (zuhur) are also used—to which all true Muslims had to respond by declaring their allegiance. In practice the latter meant an armed rebellion, hence a minimum number of armed support (nusrah) was specified and fixed at 313 followers, based on the number of Muhammad's followers at the Battle of Badr. Crucially, the da'wa was not contingent upon the popularity of the candidate or a general consensus of the Muslims; the prospective imam was compelled to rise by God, and his success depended on God alone. In more practical terms, leadership of a successful uprising against a tyrannical oppressor was proof of the imam's political and military abilities. On the other hand, it was expected that the "most excellent" candidate could be imam, hence if another, more excellent one, were to arise, the incumbent imam would be bound to surrender authority to him. Likewise, any moral transgressions or loss of the qualifying attributes rendered the legitimacy of the imamate void.

The historian Najam Haider sums up the Zaydi imamate as follows: "a qualified candidate earned followers through his scholarly and personal qualities and seized power through his military prowess. The ideal Zaydī Imām was both a 'man of the pen' and a 'man of the sword'." Unlike the Imami Shi'a, who consider their imams to be religious leaders first and foremost, vested with infallibility on matters of doctrine, the political aspects of the imamate were uniquely central to the Zaydi conception of the office: the Zaydi imams were recognized merely as knowledgeable individuals, whose judgment was potentially fallible and represented a "best guess" at God's will, and not inherently superior to the rulings of other Zaydi scholars. Only the collective consensus of the Hasanids and Husaynids could unequivocally establish the correctness or not of doctrinal issues.