Ayatullah Muhammad Taqi Misbah-Yazdi (Mesbah-Yazdi), born 1934, is a senior Iranian Twelver Shi'i religious scholar who supports the system of government instituted in the early 1980s by Grand Ayatullah al-Sayyid Ruhullah Khumayni (Ruhollah Khomeini) and his supporters after the 1978-1979 Iranian Revolution. He is often rumored to be the "spiritual adviser" to Iran's current president, the controversial Mahmoud Ahmadinejad, as well as the head of the underground Hojjatiyyeh (Hujjatiyya; Hojjatieh) movement, which was banned by Khumayni. Misbah-Yazdi denies the latter claim and no substantial evidence has been offered to support it.
The ayatullah is a member of the Assembly of Experts, a powerful government body composed of Shi'i jurists (mujtahids) who are responsible for electing and monitoring the country's supreme leader (rahbar-e jumhuri-ye Islami). Misbah-Yazdi is a staunch supporter of Khumayni's conception of wilayat al-faqih (velayat-e faqih in Persian), approximately the "authority of the jurisconsult." Khumayni argued that in the absence of the twelfth Imam, Muhammad al-Mahdi, who Twelver Shi'is believe is in a mystical hiding from which he will return at an appointed time, the 'ulama (religious scholars) were to act in his stead. Of the 'ulama (or fuqaha), a single jurist (faqih) would be the leading guide. Although Twelver Shi'i scholars have long argued that they were acting as stewards, of sort, of Twelver Shi'is until the return of the Imam, Khumayni expanded his definition of wilayah significantly and his concept was and is not accepted or considered a requirement by many Twelver Shi'i 'ulama.
In the quote below, taken from Misbah-Yazdi's official web site, he succinctly discusses the importance, in his view, of Khumayni's wilayat al-faqih (the English translation is the official one from his web site; I have also included the Arabic and Persian versions of the statement):
"The most important thing which we should defend to the last drop of our blood is vilayat faqih. This is what the arrogant powers hatch their plot and try their best to challenge. That is because, different authorities regardless of their status, position, and clothing are prone to be misled; and the only one they cannot mislead is the valy faqih. This is why the enemies of the Islamic system suffer enormous costs in order to weaken vilayat faqih."
إنّ أهمّ الامور التي يتعيّن علينا الدفاع عنها حتّى آخر قطرة دم في عروقنا هي مسألة ولاية الفقيه وإنّ كلّ مؤامرات ومساعي الاستكبار تصبّ في وادي مواجهة ولاية الفقيه. إذ من الممكن بطريق أو باُخرى حرف الشخصيّات المختلفة بأيّ زيّ تزيّت وفي أيّ منصب كانت، وإنّ الشخصيّة الوحيدة التي لا يمكن حرفها هي شخصيّة الوليّ الفقيه والقائد. ولهذا السبب نرى أنّ أعداء النظام الإسلاميّ قد بذلوا ويبذلون مبالغ طائلة في سبيل تضعيف ولاية الفقيه