Béjaïa becomes the capital of the Hammadid Dynasty in Algeria.
Third expedition of the Almoravid army in al-Andalus destined to definitely subdue the taifas kingdoms. Córdoba, Seville, Grenada, Málaga, Almería and Ronda fall to the troops of Yusuf ibn Tashfin.
Arts and culture
Troubadours begin playing in Provence.
Science and technology
Song Dynasty Chinese author Qin Guan writes the Can Shu (Book of Sericulture), which describes a silk-reeling machine that has the world's oldest known mechanical belt drive.
King William II of England invades Normandy and gains a foothold in it.
Ladislaus I of Hungary occupies Slavonia.
King Stjepan II of Croatia, the last member of the Trpimirović dynasty, died peacefully without leaving an heir.
October 17 – London Tornado of 1091: A T8/F4 tornado is recorded in St Mary-le-Bow of London, England; the storm destroys London Bridge.
Henry, the third son of William the Conqueror, is forced to surrender his property of Cotentin in Normandy after his two older brothers, William Rufus and Robert Curthose, make a peace agreement.
King Malcolm III of Scotland makes an unsuccessful attempt to invade English territory, but is finally forced to pay homage to King William II of England.
Cardiff Castle is built.
April 29 – Battle of Levounion: The Pechenegs besiege Constantinople, but are defeated so decisively by Emperor Alexius I that they fade into oblivion.
February; With the taking of Noto, the Normans complete the 30-year-long conquest of Sicily from the Islamic rulers.
The Islamic Abbadid dynasty ruling in Spain falls when the Almoravids storm Seville. Confronted with this new threat, the king of Badajoz, al-Mutawakkil ibn al-Aftas, obtains the support of Castile in exchange for the Muslim positions on the Tagus river — Sintra, Santarém and Lisbon.
Roger Guiscard takes Malta.
Athanasius VI bar Khamoro becomes Syriac Orthodox Patriarch of Antioch.
April 21 – The Diocese of Pisa is elevated to the rank of metropolitan archdiocese by Pope Urban II.
May – King William II of England annexes Cumbria from the Scottish Celtic kingdom of Strathclyde and builds Carlisle Castle.
May 9 – Lincoln Cathedral is consecrated in England.
May 21 – Synod of Szabolcs is convened in Hungary.
High tides cause great flooding in England and Scotland. The Kentish lands of Earl Godwin are inundated, becoming known as the Goodwin Sands.
The Song Dynasty Chinese scientist and statesman Su Song publishes his Xin Yi Xiang Fa Yao, a treatise outlining the construction and operation of his complex astronomical clocktower built in Kaifeng, China. It also includes a celestial atlas of five star maps.
April – Sviatopolk II becomes Grand Prince of Kiev and ruler of Kievan Rus'.
March 6 – The Frankish monk, philosopher and theologian Anselm is nominated as Archbishop of Canterbury; he is consecrated on December 4.
April 8 – Construction of Winchester Cathedral by Bishop Walkelin in England is completed.
May 26 – Battle of the Stugna River: The Cuman people defeat the princes of Kievan Rus'.
September – Magnus Barefoot is crowned king of Norway.
August 11 – Construction of Durham Cathedral in England begins.
November 13 – Battle of Alnwick: Malcolm III of Scotland, while attempting to invade England, is defeated and killed by the forces of William II of England. Malcolm's brother Donald takes the Scottish throne.
Normans occupy southern Wales, constructing Cardiff and Pembroke Castles.
Henry of Burgundy becomes Count of Portugal through his marriage to Theresa, Countess of Portugal.
Saint Canute's Cathedral is built in Odense, Denmark.
Fire causes extensive damage in London.
May – El Cid completes his conquest of Valencia, Spain, and begins his rule of Valencia. The Almoravid campaign to regain the city fails.
May 15 – The Cathedral of Saint Agatha in Catania is consecrated by the Breton Abbot Ansger of Saint Euphemia.
October 8 – St Mark's Basilica is consecrated in Venice.
November 12 – Donald III succeeds Duncan II as King of Scotland.
The city of Zagreb, Croatia, is first mentioned as a bishopric see.
Raymond IV of Toulouse becomes Count of Toulouse.
Antipope Clement III is deposed, and Urban II becomes pope.
March – Byzantine emperor Alexius I Comnenus sends ambassadors to Pope Urban II, at the Council of Piacenza, to discuss sending mercenaries against the Seljuk Turks.
July – Coloman begins to establish himself as King of Hungary, following the death of his father.
August 5 – The Valence Cathedral is consecrated in Valence, France.
November 19 – The Council of Clermont begins. The council is called by Pope Urban II to discuss sending the First Crusade to the Holy Land.
November 27 – Pope Urban II preaches the First Crusade at the Council of Clermont; Peter the Hermit begins to preach throughout France.
Overpopulation in France, according to Pope Urban II.
November 28 – On the last day of the Council of Clermont, Pope Urban II appoints Bishop Adhemar of Le Puy and Count Raymond IV of Toulouse to lead the First Crusade to the Holy Land.
Cuman invasion of Thrace in support of pretender Constantine Diogenes.
The Second County of Portugal is established for the second time by Count Henry of Burgundy. The same year, the Almoravids start pushing back the Christians to the positions they occupied a decade earlier. This offensive begins with the reconquest of Lisbon which had been given away to Castile four years before.
Pembroke Castle is built in Wales.
Bernard becomes Bishop of Brandenburg.
In Ireland, the Diocese of Waterford is erected.
The first documented teaching at the University of Oxford occurs.
In England, Norwich School is founded as an episcopal Grammar School.
The People's Crusade, the German Crusade, and the First Crusade begin. On October 21, Kilij Arslan I of the Sultanate of Rum ends the People's Crusade at the Battle of Civetot near İznik.
Peter I, King of Aragon, conquers Huesca.
Phayao, now a province of Thailand, is founded as a kingdom.
Late April – A large band of Crusaders approaches Speyer and massacred the Jewish population.
The University of Salerno is founded.
Edgar deposes Donald III and Edmund to become king of Scotland.
The First Crusade proceeds towards Palestine:
June 3 – the Norman crusaders join the rest of the army during the siege of Nicaea.
June 19 – the city of Nicaea falls to the Crusaders after a month siege.
July 1 – Crusaders win the Battle of Dorylaeum and capture Latakia from the Seljuk Turks.
October 21 – the siege of Antioch by the crusaders begins..
December 31 – at the battle of Harenc, the crusaders defeat the troops from Aleppo trying to come to the relief of besieged Antioch.
Croatian King Petar Svačić dies as the last Croatian king in the Battle of Gvozd Mountain against the army of the King Coloman of Hungary
New Almoravid campaign in al-Andalus.
The First Crusade proceeds towards Palestine
February 9 – The crusaders defeat Ridwan of Aleppo.
June 3 – After eight months of the first Siege of Antioch, the crusaders take the city.
June 5 – Kerbogha, atabeg of Mosul, leader of the Seljuq Turks, arrives at Antioch, beginning the second siege a few days later.
June 28 – Kerbogha is defeated by the crusaders at the Battle of Antioch.
December 12 – After a month's siege, the crusaders take Ma'arra and massacre part of the population.
July 14 – Donation of Altavilla: Bohemond I, the new crusader ruler of Antioch grants commercial privileges and the right to use warehouses (fondaco) and the church of Saint John to the Republic of Genoa. This marks the beginning of Italian merchant settlements in the Levant.
August – The Fatimids retake Jerusalem from the Turks.
The Byzantine Empire retakes Smyrna, Ephesus and Sardis.
June or July – In the Battle of Anglesey Sound, a fleet led by Magnus Barefoot, King of Norway, reverses an Anglo-Norman invasion of north Wales. Magnus also conquers the Orkney Islands, the Hebrides and the Isle of Man for Norway.
March 21 – Cîteaux Abbey is founded by the Cistercian Order.
Siege of Jerusalem during the First Crusade:
January 13 – Crusaders set fire to Mara, Syria.
June 7 – The First Crusade: The Siege of Jerusalem begins.
July 8 – 15,000 starving Christian soldiers march around Jerusalem.
July 15 – Christian soldiers under Godfrey of Bouillon, Robert II of Flanders, Raymond IV of Toulouse and Tancred take Jerusalem at the end of this difficult siege.
July 22 – The Kingdom of Jerusalem is founded in the Middle East.
August 12 – The Crusaders defeat the Fatimids at the Battle of Ascalon.
August 13 – Pope Paschal II succeeds Pope Urban II as the 160th pope.