- Feb 1 Space Shuttle Columbia disintegrates during reentry into the Earth's atmosphere, killing all seven astronauts aboard.
- Feb 15 An estimated 6-11 million people around the world take to the streets to protest against war with Iraq
- Mar 13 The journal Nature reports that 350,000-year-old footprints of an upright-walking human have been found in Italy
- Mar 19 Invasion of Iraq by American and British led coalition begins without United Nations support and in defiance of world opinion
- Apr 9 Baghdad falls to U.S. forces ending the invasion of Iraqi but resulting in widespread looting
- Apr 14 The Human Genome Project is completed with 99% of the human genome sequenced to an accuracy of 99.99%
- Apr 16 The Treaty of Accession is signed in Athens admitting 10 new member states to the European Union.
- Aug 11 NATO takes over command of the peacekeeping force in Afghanistan, marking its first major operation outside Europe in its 54-year-history.
- Dec 13 Former Iraqi President Saddam Hussein is captured near his home town of Tikrit, during Operation Red Dawn by US forces
The Spirit Rover is launched, beginning NASA's Mars Exploration Rover mission
On June 10, 2003
- Apr 29 Barbaro, American thoroughbred racehorse (d. 2007)
- Apr 29 Maud Behn, Princess of Norway
- Jul 4 Alessia di Matteo, Italian medical figure (d. 2005)
- Aug 20 Prince Gabriel of Belgium
- Aug 24 Alexandre Coste, son of Albert II, Prince of Monaco