According to the FBI, Al-Zawahiri is [was] a physician and the founder of the Egyptian Islamic Jihad (EIJ). This organization opposes the secular Egyptian Government and seeks its overthrow through violent means. In approximately 1998, the EIJ led by Al-Zawahiri merged with Al Qaeda.

https://www.fbi.gov/wanted/wanted_terrorists/ayman-al-zawahiri

Zawahiri was closely affiliated with both the bombing of the USS Cole in October 2000 and the attacks of September 11, 2001. Zawahiri gradually became al-Qaeda’s chief spokesman, issuing commentary and threats on a range of issues over the years. In 2009 the Department of State determined that Zawahiri appeared to be al-Qaeda’s leading decision maker, while bin Laden reportedly occupied figurehead status.

Zawahiri was appointed leader of al-Qaeda in 2011 following bin Laden’s death.