Here's a look at the grand ayatollah. The 90-year-old al-Sistani is the top Shiite cleric in Iraq and a powerhouse in the Shiite world. Millions of Shiites the world over revere him as a spiritual guide, turning to him on all sorts of questions from day-to-day matters to complex theological issues.

Who is the current Shia Imam?

The twelfth and final Imam is Muhammad al-Mahdi, who is believed by the Twelvers to be currently alive, and hidden in the Major Occultation until he returns to bring justice to the world.

What is Ayatollah in Iran?

Ayatollah (UK: /ˌaɪəˈtɒlə/ or US: /ˌaɪəˈtoʊlə/; Persian: آیت‌الله‎, romanized: āyatollāh) is an honorific title for high-ranking Twelver Shia clergy in Iran and Iraq that came into widespread usage in the 20th century.