Yassin studied at Al-Azhar University in Cairo, Egypt following secondary school, despite his paralysis as a result of a childhood injury. The Muslim Brotherhood was founded at Al-Azhar, and the University was a hotbed of Islamism and Arab nationalism. Yassin joined the Brotherhood at some point during his studies at Al-Azhar.
Yassin has repeatedly said that the land of Israel is "consecrated for future Muslim generations until Judgement Day. and that "The so-called peace path is not peace and it is not a substitute for jihad and resistance." .
At least since taking over the reigns of Hamas, Yassin has preached in favor of violence against the state of Israel and its citizens.
In 1989 Yassin allegedly ordered the killing of Palestinians who he believed had collaborated with the Israel Defence Forces. He was also convicted of ordering the kidnapping and killing of two IDF soldiers. He was arrested by Israel for these crimes and sentenced to life imprisonment by an Israeli court.
In 1996 IDF Brigadier General David Agmon wrote that Yassin has "defined [Hamas'] political, social, and spiritual values while also addressing the military aspect." Agmon wrote that Yassin operates "terrorist squads, gives approval for assassinations and murders, recruis activists and raises funds [while] procuring weapons and smuggling them into the Occupied Territories." 
In 1997 Yassin was released from Israeli prison as part of an arrangement with Jordan following a failed assassination attempt of Khaled Mashal conducted by the Israeli Mossad in Jordan. Yassin was released by Israel in exchange for two Mossad agents who had been arrested by Jordanian authorities.
Following his release, Yassin returned to his role as spiritual leader of Hamas. He immediately resumed his calls for assaults on Israel using tactics including suicide bombings against both Israeli military and civilian targets.
During the various stages of the "peace process" between Israel and the Palestinian Authority, Yassin has been repeatedly placed under house arrest by the PA. Each time Yassin has been placed under house arrest he has eventually been released, often after extended demonstrations by his supporters.
Three months later, on September 6, 2003, an Israeli Air Force (IAF) F-16 dropped a quarter-ton bomb on a building in Gaza City, the Gaza Strip. Yassin was in the building at the time and was lightly wounded by the assassination attempt. Israeli officials would later confirm that Yassin was a target of the attack. Yassin was treated for injuries sustained in the attack at Shifa Hospital in Gaza City.
Following the assassination attempt, Yassin told reporters that "Days will prove that the assassination policy will not finish the Hamas. Hamas leaders wish to be martyrs and are not scared of death. Jihad will continue and the resistance will continue until we have victory, or we will be martyrs." 
Yassin further said that Hamas would teach Israel an "unforgettable lesson" as a result of the assassination attempt .
See also: Abdel Aziz al-Rantissi