The Iranian Revolution, which transforms Iran from a monarchy under Shah Mohammad Reza Pahlavi to an Islamic republic under Ayatollah Ruhollah Khomeini, the leader of the revolution and founder of the Islamic Republic, has been called "the third great revolution in history," following the French and Bolshevik revolutions, and an event that "made Islamic fundamentalism a political force ... from Morocco to Malaysia."

Although some might argue that the revolution is still ongoing, its time span can be said to have begun in January 1978 with the first major demonstrations to overthrow the Shah, and concluded with the approval of the new theocratic Constitution — whereby Khomeini becomes Supreme Leader of the country — in December 1979.

In between, Mohammad Reza Pahlavi flees Iran in January 1979 after strikes and demonstrations paralyze the country, and on February 1, 1979 Ayatollah Khomeini returns to Tehran to a greeting by several million Iranians.

The final collapse of the Pahlavi dynasty occurs shortly after on February 11 when Iran's military declares itself "neutral" after guerrillas and rebel troops overwhelmed troops loyal to the Shah in armed street fighting.

Iran officially becomes an Islamic Republic on April 1, 1979 when Iranians overwhelmingly approve a national referendum to make it so.The revolution is unique for the surprise it creates throughout the world: it lacks many of the customary causes of revolution — defeat at war, a financial crisis, peasant rebellion, or disgruntled military; it produces profound change at great speed; it has overthrown a regime thought to be heavily protected by a lavishly financed army and security services; and it has replaced an ancient monarchy with a theocracy based on Guardianship of the Islamic Jurists (or velayat-e faqih).

Its outcome — an Islamic Republic "under the guidance of an 80-year-old exiled religious scholar from Qom" — is, as one scholar put it, "clearly an occurrence that had to be explained.…"