The Imams of Yemen and later also the Kings of Yemen were religiously consecrated leaders belonging to the Zaidiyyah branch of Shia Islam. They established a blend of religious and political rule in parts of Yemen from 897. Their imamate endured under varying circumstances until the republican revolution in 1962, then the formal abolition of the monarchy in 1970. Zaidiyyah theology differed from Ismailis or Twelver Shi'ites by stressing the presence of an active and visible imam as leader. The imam was expected to be knowledgeable in religious sciences, and to prove himself a worthy headman of the community, even in battle if this was necessary. A claimant of the imamate would proclaim a "call" (da'wa), and there were not infrequently more than one claimant. The historian Ibn Khaldun (d. 1406) mentions the clan that usually provided the imams as the Banu Rassi or Rassids (Arabic: الرسيين). In the original Arab sources the term Rassids is otherwise hardly used; in Western literature it usually refers to the Imams of the medieval period, up to the 16th century. The Rassid branch that came to power with imam al-Mansur al-Qasim (r. 1597-1620) is known as Qasimids (Al al-Qasimi).

The imams based their legitimacy on descent from the Islamic prophet Muhammad, mostly via the prominent Zaydiyya theologian al-Qasim al-Rassi (d. 860) - his cognomen refers to ar-Rass, a property in the vicinity of Mecca that he owned. After him, the medieval imams are sometimes known as Rassids. The first of the ruling line, his grandson al-Hadi ila'l-Haqq Yahya, was born in Medina. His fame as an intellectual as well as a leader of note, led to his invitation to Yemen. He was summoned to govern the highland tribes in 893 and again in 896-97. Al-Hadi introduced a multitude of policies and practices that evolved into the particular Yemeni Zaidi Shia brand. The efforts of al-Hadi eventually became the basic guidelines for the religious as well as political characteristics of Yemeni Zaydism. Al-Hadi, however, was not able to consolidate his rule in all of Yemen. He could not even create an enduring state in the highlands, due to the strong localism persisting in the region. There were revolts as well as segments of the population that did not accept his and his successors' pretensions to religio-political rule.

Although he did not succeed in establishing any permanent administrative infrastructure, al-Hadi's descendants became the local aristocracy of the northern highlands, and it is from among them that most of the imams of Yemen were selected for the next one thousand years. Occasionally the imams were drawn from other lines descending from Muhammad.

Yemen throughout most of that period was only rarely a unified political entity; in fact, what was included within its frontiers varied widely, and it has not been governed consistently or uniformly by any single set of rulers except for brief periods. It existed as a part of a number of different political systems/ruling dynasties between the ninth and sixteenth centuries, after which it became a part of the Ottoman Empire.

After Imam al-Hadi's death in 911, his sons took over the imamate in turn, although it was not hereditary but rather elective among the descendants of Muhammad. From the 11th to the early 17th centuries, however, the imams were usually not chosen from the sons of the former imam, but rather circulated among the various Rassid branches. Meanwhile, a multitude of smaller dynasties and families established themselves in the highlands, as well as in Tihama (the low coastal plain) where the imams rarely ruled. Among the better known of these are the Yu'firids (in San'a and Shibam, 847-997), the Sulayhids (in the southern highlands, 1047-1138), the Zuray'ids (in Aden, 1080-1174), and the Hatimids (in San'a, 1098-1174). It was during this period, when the Fatimid state was influential, that a portion of the population was converted to Isma'ili Shiʿism.

Beginning with the conquest of Yemen by the family of Salah al-Din ibn Ayyub (Saladin) in 1174, a series of dynasties exercised a modicum of control and administration in Yemen for roughly the next 400 years; these are, in chronological sequence, the Ayyubids, from 1173/74 to 1229; the Rasulids, from 1229 to 1454; the Tahirids, from 1454 to 1517; and the Mamluks, from 1517 to 1538, when the Ottoman Empire took the Yemeni Tihama.

During most of this period, the dynasties and their rulers were primarily engaged in familial, regional, and occasionally sectarian disputes. Ironically, the Sunni Rasulids, who eventually concentrated their rule in southern Yemen for precisely that reason, were the dynasty under which the region experienced the greatest economic growth and political stability.

For part of the medieval era the Zaydiyyah imams were eclipsed by the lowland dynasties, and for long periods there would be no imam at all (especially in 1066-1138 and 1171-1187). From the end of the thirteenth century the political fortunes of the Zaydiyya imams revived somewhat. They were able to hold their own against the Rasulids and Tahirids and sometimes expand their territory. Often, however, and especially after 1436, the imamate was split between several contenders.