October 18 is the 291st day of the year (292nd in leap years) in the Gregorian calendar. There are 74 days remaining until the end of the year.
* 1009 – The Church of the Holy Sepulchre, a Christian church in Jerusalem, is completely destroyed by the Fatimid caliph Al-Hakim bi-Amr Allah, who hacks the Church's foundations down to bedrock.
* 1016 – The Danes defeat the Saxons in the Battle of Ashingdon.
* 1081 – The Normans defeat the Byzantine Empire in the Battle of Dyrrhachium.
* 1210 – Pope Innocent III excommunicates German leader Otto IV.
* 1356 – Basel earthquake, the most significant historic seismological event north of the Alps, destroyed the town of Basel, Switzerland.
* 1386 – Opening of the University of Heidelberg
* 1561 – Fourth Battle of Kawanakajima – Takeda Shingen defeats Uesugi Kenshin in the climax of their ongoing conflicts.
* 1599 – Michael the Brave, Prince of Wallachia, defeats the Army of Andrew Bathory in the Battle of Şelimbăr, leading to the first recorded unification of the Romanian people.
* 1648 – Boston Shoemakers form first U.S. labor organization.
* 1748 – Signing of the Treaty of Aix-la-Chapelle ends the War of the Austrian Succession.
* 1767 – Mason-Dixon line, survey separating Maryland from Pennsylvania is completed.
* 1775 – African-American poet Phillis Wheatley freed from slavery.
* 1851 – Herman Melville's Moby-Dick is first published as The Whale by Richard Bentley of London.
* 1860 – The Second Opium War finally ends at the Convention of Peking with the ratification of the Treaty of Tientsin, an unequal treaty.
* 1867 – United States takes possession of Alaska after purchasing it from Russia for $7.2 million. Celebrated annually in the state as Alaska Day.
* 1898 – United States takes possession of Puerto Rico.
* 1912 – The First Balkan War begins.
* 1914 – The Schoenstatt Movement is founded in Germany.
* 1921 – The Crimean Autonomous Soviet Socialist Republic is formed as part of the RSFSR.
* 1922 – The British Broadcasting Company (later Corporation) is founded by a consortium, to establish a nationwide network of radio transmitters to provide a national broadcasting service.
* 1925 – The Grand Ole Opry opens in Nashville, Tennessee.
* 1929 – Women are considered "Persons" under Canadian law.
* 1936 – Adolf Hitler announces the Four Year Economic Plan to the German people. The plan details the rebuilding of the German military from 1936 to 1940.
* 1944 – Adolf Hitler orders the establishment of a German national militia.
* 1944 – Soviet Union invades Czechoslovakia.
* 1944 – Adolf Hitler orders the public funeral procession of Nazi field Marshall Erwin Rommel, commander of the Deutsches Afrika Korps
* 1945 – The USSR's nuclear program receives plans for the United States plutonium bomb from Klaus Fuchs at the Los Alamos National Laboratory.
* 1945 – A group of the Venezuelan Armed Forces, led by Mario Vargas, Marcos Pérez Jiménez and Carlos Delgado Chalbaud, staged a coup d'état against then president Isaías Medina Angarita, who is overthrown by the end of the day.
* 1954 – Texas Instruments announces the first Transistor radio.
* 1964 – The 1964-1965 New York World's Fair closes for its first season after a six-month run.
* 1967 – The Soviet probe Venera 4 reaches Venus and becomes the first spacecraft to measure the atmosphere of another planet.
* 1968 – The U.S. Olympic Committee suspends Tommie Smith and John Carlos for giving a "black power" salute during a victory ceremony at the Mexico City games.
* 1968 – Bob Beamon sets a world record of 8.90 m in the long jump at the Mexico City games.
* 1977 – German Autumn: a set of events revolving around the kidnapping of Hanns-Martin Schleyer and the hijacking of a Lufthansa flight by the Red Army Faction (RAF) comes to an end when Schleyer is murdered and various RAF members allegedly commit suicide.
* 1989 – East German leader Erich Honecker resigns.
* 1991 – Azerbaijan declares independence from USSR.
* 2003 – Bolivian Gas War: President Gonzalo Sánchez de Lozada, is forced to resign and leave Bolivia.
* 2007 – Karachi bombings: attempted assassination of Benazir Bhutto.