Occultation (Arabic: غَيْبَة, ghayba) in Shia Islam refers to the eschatological belief that the Mahdi, a descendant of the Islamic prophet Muhammad, has already been born and he was subsequently concealed, but he will reemerge and he will establish justice and peace on earth at the end of time. The signs of his (re)appearance are largely common in Shia and Sunni (although Sunni do not believe the Mahdi has already been born and is in occultation), and the belief in the eschatological Mahdi remains popular among all Muslims, possibly owing to numerous traditions to this effect in canonical Sunni and Shia sources.

However, the branches of Shia Islam that believe in it differ with regard to the identity of the Mahdi. The mainstream Shia identifies him as Muhammad al-Mahdi, the twelfth imam, who is believed to be responsible for the affairs of men and, in particular, their inward spiritual guidance during the occultation.

Twelver Shia is the mainstream branch of Shia Islam, accounting for 85 percent of the Shia population. The Twelvers believe that their twelfth Imam, Muhammad al-Mahdi, is in occultation. During his Minor Occultation (874–941), the twelfth Imam is believed to have remained in regular contact with four successive agents, collectively known as the Four Deputies (al-nuwwab al-arba'). During the Major Occultation (941–present), however, there is no agent of the Hidden Imam on earth, though it is believed that he remains providentially living in his physical body until his reappearance in the end of time.

The Twelver theory of occultation crystallized in the first half of the fourth (tenth) century based on rational and textual arguments. This theory, for instance, sets forth that the life of Muhammad al-Mahdi has been miraculously prolonged, arguing that the earth cannot be void of the imam as the highest proof (hujja) of God. As another example, while the Abbasid threat might have initially forced the twelfth imam into occultation, according to this doctrine, his absence continues until initial conditions are met for his reappearance, including humankind's readiness for the message of the Hidden Imam.

Immediately after the death of Hasan al-Askari in 260 (873–874), Uthman al-Amri (d. 874–875) claimed that the eleventh Imam had a young son, named Muhammad, who had entered a state of occultation due to the threat to his life from the Abbasids. As the special representative of al-Askari, Uthman also claimed that he had been appointed to represent the son of the eleventh Imam. Possibly the only public appearance of Muhammad was to lead the funeral prayer for his father instead of his uncle, Ja'far.

In his new capacity, Uthman received petitions and made available their responses, sometimes in writing. As the closest associate of al-Askari, most of al-Askari's local representatives continued to support Uthman. He later introduced his son, Abu Ja'far Muhammad ibn Uthman (d. 916–17), as the next representative of al-Mahdi. In turn, as his replacement, Abu Ja'far nominated Abu al-Qasim al-Husayn ibn Ruh al-Nawbakhti (d. 937–38).

This period, later termed the Minor Occultation (al-ghaybat al-sughra), ended after about seventy years with the death of the fourth agent, Abu al-Hasan Ali ibn Muhammad al-Samarri (d. 940–41), who is said to have received a letter from al-Mahdi shortly before his death. The letter predicted the death of Abu al-Hasan in six days and announced the beginning of the complete (tamma) occultation, later called the Major Occultation (al-ghaybat al-kubra). The letter, ascribed to al-Mahdi, added that the complete occultation would continue until God granted him permission to manifest himself again in a time when the earth would be filled with tyranny. This and similar letters to the four agents and other Shia figures are said to have had the same handwriting, suggesting that they were written by the Hidden Imam.

The Major Occultation, a later term, began with the death of the fourth agent in 329 (940–941), who did not designate a successor. In this period, which continues today, there is no agent of the Hidden Imam on earth. There were likely early traditions among the Shia that had already predicted the two periods of occultation. These hadiths were previously cited, for instance, by the Waqifites in reference to the two arrests of Musa al-Kazim, the seventh Imam.