In this video image provided by the Site Intelligence Group on April 15, Ayman al-Zawahiri delivers a speech encouraging Muslims to rise up against both NATO and Moammar Gaddafi's forces in Libya. (AFP via GETTY IMAGES)

Zawahiri, who was trained as a surgeon and worked for the Red Crescent Society, came in contact with bin Laden in Peshawar, Pakistan, in 1980. A year later, he was imprisoned for taking part in the assassination of Egyptian President Anwar Sadat. Following his release, Zawahiri quickly rose to lead the Egyptian Islamic Jihad, which according to the United States was directly involved in the 1998 bombings of American Embassies in Dar es Salaam, Tanzania, and Nairobi, Kenya.

Like bin Laden, Zawahiri has released numerous audio and video messages and has vocally expressed his ideology since Sept. 11, 2001, threatening the United States and other Western countries. In one of his first videos (see below), he praised Allah for the attack on the twin towers.

Here are five important things you should know about the man poised to lead al-Qaeda:

1. Born in 1951 in Egypt to a family of doctors; graduated from Cairo University’s medical school and earned a degree in surgery in 1978.

2. Arrested at age 15 for being involved in the Muslim Brotherhood in Egypt.

3. Inspired by the ideologies of Sayyid Qutb, whom many consider the godfather of modern-day radical Islam.

4. Zawahiri uses several aliases, the most common of them being Abu Muhammad, Abu Fatima, Abu al-Mu’iz, “the doctor” and “the teacher.”

5. Considered the brain behind bin Laden and al-Qaeda’s terror operations.

Although his whereabouts remain unclear, Zawahiri is believed to be hiding in the same region as bin Laden was, and the FBI is offering up to $25 million for any information that leads to his capture.