June 19: Dutch Wikipedia
- January 15: Wikipedia begins
- January 16: First edit is made to the main page
- date NA but prior to November (NPOV) Neutral Point of View is formally introduced as Wikipedia's core non-negotiable editorial policy
- November: Meta-Wikipedia (now simply meta) was created. See also, en:WP:Meta.
- February 12: English Wikipedia reaches 1,000 articles
March NA: Slovenian Wikipedia
- January 25: English Wikipedia switched to Phase II software, the precursor to MediaWiki; Jimmy Wales declares it "Magnus Manske Day", for the coder who released the update.
- February NA: Participants of the Spanish Wikipedia "fork" and establish the Enciclopedia Libre
- July 20: The English Wikipedia switched to MediaWiki, then known as the Phase III software.
- August NA: URL is changed from wikipedia.com to wikipedia.org
- August 27: The German Wikipedia introduces the concept of "Featured articles" (German: "Exzellente Artikel"). The first article to be promoted to featured article status is the article "Mark Twain" on August 27, 2002. The English Wikipedia adopts the concept in October 2003. An early copy of Wiktionary can be found here (Web Archive
- December 12: Wiktionary, the first Wikipedia sister project is created. The pages created the first day include the talk page of the main page, dictionary, dog, free, What Wiktionary is not. And perhaps the first ever created page was the Home page
- December 27: First real life meetup of Wikipedians in Berlin at the Chaos Communication Congress
- September NA: Finnish Wikipedia reaches 1,000 articles
February 16: Serbian Wikipedia
- April 23: Second real life meetup of Wikipedians in Warsaw, Poland
- June 20: The Wikimedia Foundation is founded
- October 12: The finalized Wikipedia puzzle logo is adopted
- October 28: Foundation of the first Wikipedia-"Stammtisch" in Munich, Germany
- December 8: Wikipedia:Mediation Committee is created by Jimmy Wales
- January 22: English Wikipedia reaches 100,000 articles
- January 24: German Wikipedia reaches 10,000 articles
January 7: Kurdish Wikipedia
- January 10: The articles "Krautinsel" and "Herrenchiemsee" were the first spoken articles in the German Wikipedia. A project page on spoken articles started at January 20.
- February 22: On the English Wikipedia, the first "Did you know" article, the pencil sharpener, is posted to the main page. (See diff.)
- May 12: Wikipedia wins the Annual Webby Award in the "Community" category
- May 14: The first "Picture of The Day" is posted to the main page of the English Wikipedia
- May 30: The categorization scheme is introduced to Wikipedia articles
- October: The German Wikipedia starts its first writing competition (German: "Schreibwettbewerb"). Between October 2004 and September 2010, more than 500 articles participating in the competition reach "featured" and "good" article status
- October 14: Directmedia Publishing, a German publishing house, produces the first Wikipedia version on CD.
Formal Chapters Created
- June 13: Wikimedia Germany
- October 23: Wikimedia France
- January NA: English Wikipedia reaches 200,000 articles
- February 25: Wikipedia reaches 500,000 articles in 50 languages
- April 20: English Wikipedia reaches 250,000 articles
- June 9: Portuguese Wikipedia reaches 10,000 articles
- June 13: In the night before the chapter founding the German Wikipedia reaches 100,000 articles (with "Opel Laubfrosch" was nominated as #100,000 by Jimbo Wales in Berlin)
- September 20: Wikipedia reaches 1,000,000 articles in 105 languages
- November 16: Catalan Wikipedia reaches 10,000 articles
- November 20: English Wikipedia reaches 400,000 articles
February NA: Armenian Wikipedia (Armenian (Հայերեն))
- January 10: The first issue of the English Wikipedia Signpost, a newspaper for the Wikipedia community, is published.
- March 22: The German Wikipedia introduces the concept of "Good articles". The first article being marked as good article is "Fußball-Weltmeisterschaft 2006" (engl. 2006 FIFA World Cup)
- June 26: Wikimedia Germany yearly meeting in c-base, Berlin
- October 14: The first German Wikipedia version on DVD was produced by Directmedia Publishing (after the CD version in October 2004).
- Aug 4-8: First Wikimania is held in Frankfurt, Germany
- December 5: User registration becomes obligatory for the creation of new articles in the English Wikipedia, as a result of the Wikipedia biography controversy
- December 20: "Languageicon" is introduced to unify language icon templates
- December 22: English Wikipedia:Semi-protection policy is implemented
- December 25: cite.php (the citation and referencing system) added .
- This one extension possibly more than any other caused a seismic shift in quality within the year. Prior to 2005 most "ordinary" articles had occasional "ref" tags or inline bare hyperlinks and their maintenance was patchy. Cite.php meant that even ordinary articles were routinely bristling with formatted cites in a standard location within a year or two.
- date NA: First formal fundraiser raises $94,000 for system upgrades
- date NA: "Semi-protection" feature is introduced
Formal Chapters Created
- July 17: Wikimedia Italy
- August 14: Wikimedia Poland
- December 3: Wikimedia Serbia
- March 18: English Wikipedia reaches 500,000 articles
- May 21: Portuguese Wikipedia reaches 50,000 articles
- June 5: Korean Wikipedia reaches 10,000 articles
September 30: Pangasinan Wikipedia
- January 10: Wikipedia® becomes a registered trademark
- February 25: Wikimedia Germany yearly meeting in Haus der Jugend, Frankfurt/Main
- February 28: The millionth user account is registered for the English language edition
- March 17: The first Wikipedia exhibition opens at the Göttingen University Library, Germany
- April 1: The featured article seen on the Main Page ("Spoo") is a factually accurate, yet unusual article, starting a new April Fools' Day tradition on Wikipedia
- June 16/17: The first Wikipedia Academy takes place in Göttingen, Germany
- June 17: The first Wikipedia Day in Switzerland takes place at the Swiss Federal Institute of Technology Zurich
- August 4-6: The second Wikimania is held in Boston, Massachusetts, USA
- August 19: A private party of the User jcornelius in Berlin became one of the biggest community events of the German Wikipedia. More than 70 wikipedians joined the party. In the following years 2007 and 2008 he continued with it with great success.
- Augest 26-27: The first Chinese Wikimedia Conference 2006 was held in Hong Kong.
- October: The German Wikipedia establishes editorial boards for specific topic areas (German: "Redaktionen"). The Redaktionen act as contact points for people who have subject-related questions and they also systematically monitor and improve the quality of articles in their topic area. Other than WikiProjects, Redaktionen have quality management pages, where Wikipedians, who are both members of the editorial board and admins can delete articles without going through the normal request for deletion process
- November 27: First real life meetup of Wikipedians in Macedonia
- November 29: Russian Wikipedia wins the Russian national Internet award, the Runet Prize, in the Science and Education category
- December 8: Wikimedia Foundation releases to public its first ever financial audit, covering years 2004-2006
- December NA: Japanese Wikipedia wins the Web of the Year, 2006 from Yahoo! Japan Internet Guide
Formal Chapters Created
- May 14: Wikimedia Switzerland
- March 27: Wikimedia Netherlands
- January 23: Portuguese Wikipedia reaches 100,000 articles
- January NA: Wikipedia exceeds 1 billion words in all languages
- February 12: Korean Wikipedia reaches 20,000 articles
- March 1: English Wikipedia reaches 1,000,000 articles
- March 8: Spanish Wikipedia reaches 100,000 articles
- August 16: Russian Wikipedia reaches 100,000 articles
- November 24: English Wikipedia reaches 1,500,000 articles
- November 23: German Wikipedia reaches 500,000 articles
- November 12: Chinese Wikipedia reaches 100,000 articles
- November 27: French Wikipedia reaches 400,000 articles
- November 29: Portuguese Wikipedia reaches 200,000 articles
November 24: Bikol Wikipedia
- May 4-5: The second Wikipedia Day in Switzerland takes place at the University of Lugano
- June NA: The German article project on renewable raw materials (German: "Projekt Nachwachsende Rohstoffe") started as the first project funded by a national ministry. It ended in April 2010.
- August 3-5: Wikimania 2007 in Taipei
- August 24: The second German Wikipedia Academy takes place in Mainz. At the event, Wikimedia Germany and the Mainz Academy of Sciences and Literature introduce the "Zedler medal" for distinguished encyclopedic contributions. The 2007 Academy in Germany is the first Wikipedia related event being sponsored by a national government (the German Bundesministerium für Bildung und Forschung).
- September 29: The third Wikipedia Day in Switzerland takes place at the University of Teacher Education Bern
- October NA: The German Wikipedia introduces the first mini-grant program for Wikipedia authors (German "Literaturstipendium"): Wikipedians can apply for a grant to buy a book that they need for writing articles on a specific topic; the first book was bought in November ("The Biology and Evolution of Fossil Plants"). By 2010, more than 1,500 articles have been written based on books funded by this project.
- December 30: First Malayalam Wiki Meetup held in Thrissur, Kerala
- date NA: 5 million registered editor accounts.
- date NA: Wikipedia reaches 7.5 million articles in approximately 250 languages
Formal Chapters Created
- Feb 11: Wikimedia Taiwan
- June 23: Wikimedia Norway
- July 14: Wikimedia Hong Kong
- July 15: Wikimedia Israel
- October 20: Wikimedia Sweden
- December 11: Wikimedia Argentina
- June 22: Macedonian Wikipedia reaches 10,000 articles
- September 9: English Wikipedia reaches 2,000,000 articles
- October 10: Portuguese Wikipedia reaches 300,000 articles
- October 17: Malayalam Wikipedia reaches 1,00,000 edits
- October 20: Tagalog Wikipedia reaches 10,000 articles
- November 18: Spanish Wikipedia reaches 300,000 articles
- December 12- Malayalam Wikipedia reaches 5,000 articles
- December 14: Korean Wikipedia reaches 30,000 articles
November 24: Egyptian Arabic Wikipedia
- January 10: Multilingual portal at Wikipedia.org replaces direct link to English-language Wikipedia
- May 6: German Wikipedia implements "flagged revisions"
- July 16: First "featured pictures" is introduced to the Main Page of the English Wikipedia
- July 17-19: Wikimania 2008 in Alexandria, Egypt
- December 4: Wikimedia Commons received more than 80,000 files from a donation from the German Federal Archives to use in the wikipedia projects. Other projects focussed on picture grants followed in the forthcoming years.
Formal Chapters Created
- March 6: Wikimedia Czech Republic
- May 2: Wikimedia Austria
- August 6: Wikimedia Australia
- September 5: Wikimedia Indonesia
- September 27: Wikimedia Hungary
- November 5: Wikimedia United Kingdom
- November 11: Wikimedia Russia
- January 4: Korean Wikipedia reaches 50,000 articles
- January 11: Romanian Wikipedia reaches 100,000 articles
- March NA: Wikipedia reaches 10,000,000 articles in all languages
- March 28: Ukrainian Wikipedia reaches 100,000 articles
- May 11: Malayalam Wikipedia crosses Page Depth of 100
- May 14: Polish Wikipedia reaches 500,000 articles
- June 22: Portuguese Wikipedia reaches 400,000 articles
- June 25: Japanese Wikipedia reaches 500,000 articles
- July 16: Wikimedia Commons reaches 3,000,000 uploaded files
- July 18: Russian Wikipedia reaches 300,000 articles
- August 8: Malayalam Wikipedia reaches 5,000 uploaded files, Top among the Indian Wikipedias
- October 3: Italian Wikipedia reaches 500,000 articles
- November 1: Tagalog Wikipedia reaches 20,000 articles
- November 5: Macedonian Wikipedia reaches 20,000 articles
- November 18: Korean Wikipedia reaches 80,000 articles
- November 30: Dutch Wikipedia reaches 500,000 articles
- December 4: A massive upload of ~100,000 images donated by the German Federal Archive to Wikimedia Commons, the single largest donation to date.
- December 13: Norwegian bokmål Wikipedia reaches 200,000 articles
- December NA: Swedish Wikipedia reaches 300,000 articles
December 24: Picard Wikipedia
- January 16: Italian Wikipedia wins the Premiolino award
- February NA: Wikipedia:Wikipedia Loves Art, a scavenger hunt and free content photography contest among museums and cultural institutions worldwide happens aimed at illustrating Wikipedia articles, coordinated by the Brooklyn Museum
- February 26: Wikipedia starts testing the "Books" feature, a functionality to print books from collections of Wikipedia articles
- April NA: The German writing competition (German: "Schreibwettbewerb") takes place for the 10th time – TUI sponsors a journey to Turkey for the winner.
- June 22: "Wiki Loves Art" event in Netherlands. The project is run by Creative Commons Netherlands and Wikimedia Netherlands, in collaboration with the Amsterdam Museum Night Foundation and the Netherlands Institute for Heritage
- August 26-28: Wikimania 2009, Buenos Aires
- August NA: The Wikipedia Mobile iPhone appplication is announced
- September 21: Catalan Wikipedia reaches 200,000 articles
- October 13: OpenMoko launches the WikiReader, a dedicated reader device with an offline copy of the entire English Wikipedia (without images) stored on a chip
- October 13: Polish Wikipedia is cited for "special recognition for social innovation" at the 2009 Jan Łukasiewicz Awards
- October 26: A custom Google search extension for Wikipedia created
Formal Chapters Created
- March 14: Wikimedia Denmark
- May 31: Wikimedia Ukraine
- July 7: Wikimedia New York City
- June 14: Wikimedia Finland
- September 21: Wikimedia Portugal
- April 25: Malayalam Wikipedia reaches 10,000 users
- April 29: Macedonian Wikipedia reaches 30,000 articles
- May 8: Alemannic Wikipedia reaches 5,000 articles
- May 25: Arabic Wikipedia reaches 100,000 articles
- June 1: Malayalam Wikipedia reaches 10,000 articles
- June 4: Korean Wikipedia reaches 100,000 articles
- August 5: Spanish Wikipedia reaches 500,000 articles
- August 12: Portuguese Wikipedia reaches 500,000 articles
- August 17: English Wikipedia reaches 3,000,000 articles
- August NA: All Wikipedia languages reaches 14 million articles
- October 21: Malayalam Wikipedia reaches reaches 5,00,000 edits
- December 25--Christmas Day: Hungarian Wikipedia reaches 150,000 articles
- December 27: German Wikipedia reaches 1,000,000 articles
March 19: Karachay-Balkar Wikipedia
- By 2010 researchers were no longer comparing Wikipedia's science articles to other encyclopedias, but to scientific reference databases. Peer reviewed studies compared English Wikipedia's entries on drugs (2008), toxicology (2009) and cancers (2010) to reputable media and science databases. Wikipedia's drug coverage had no errors compared to 4 in the professional database, although errors of omission were more common, toxicology coverage was rated the second most accurate of all US national newspapers and media outlets, and coverage of cancers "had similar accuracy and depth to the professionally edited database" for both common and uncommon cancers.
- January 2: Catalan Wikipedia scores first in the List of Wikipedias by sample of articles, the first one to displace English wikipedia from the top position
- January 12–13: Under the name WikiWars the first Wikipedia conference focussed on a "Critical Point of View" (CPOV) took place in Bangalore, India. A second followed in Amsterdam on March 26–27 and a third in Leipzig on September 25–26.
- March 11: The British Museum is the first organization to incorporate work with a "volunteer Wikipedian in residence"
- March 21: First Malayalam Wiki Academy held in Bangalore
- April 16: 1 billionth edit.
- April 17: Third Malayalam Wiki Meetup held in Ernakulam, Kerala and releases Wiki C.D including 500 articles. This is the sixth among the world and first among a non latin language
- May 13: Updated interface is introduced, including an updated logo
- June, 4–6: Skillshare in Lüneburg organized by skillshare e.V. became the biggest community event of the German Wikipedia 2010.
- July 9: Wikimania 2010 Gdansk, Poland
- August: Wikipedia Ambassador Program introduced. Initially, the Ambassador Program consists of Campus Ambassadors and Online Ambassadors, who support students and instructors who use Wikipedia in university classrooms. The first Wikipedia Ambassador Training takes place in Washington D.C., on August 17th.
- October (whole month): In a co-operation with the German Central Library for the Blind (German: Deutsche Zentralbibliothek für Blinde, DZB) the articles of the day of the whole month were presented in a high quality spoken version. In December Wikimedia Germany published these versions on an audio CD ()
- October 1: Start of the RENDER project, the first project with involvement of an European chapter (the German) funded by the European Union.
- December 14: Developer Tim Starling finds the original backups, long believed lost, of every change made to Wikipedia from its creation in January 2001 to August 17, 2001.
Formal Chapters Created
- February 1: Wikimedia Macedonia
- April 12: Wikimedia Philippines
- July 25 or October 6: Wikimedia Estonia
- January 10: Hebrew Wikipedia reaches 100,000 articles
- February 17: Czech Wikipedia reaches 150,000 articles
- February 25: Russian Wikipedia reaches 500,000 articles
- March 28: Chinese Wikipedia reaches 300,000 articles
- March 29: Finnish Wikipedia reaches 200,000 articles
- April 7: Ukrainian Wikipedia reaches 200,000 articles
- April 17: Macedonian Wikipedia reaches 40,000 articles
- May 12: Polish Wikipedia reaches 700,000 articles
- June 22: Italian Wikipedia reaches 700,000 articles
- July 20: Tagalog Wikipedia reaches 30,000 articles
- August 17: Portuguese Wikipedia reaches 600,000 articles
- August 31: Japanese Wikipedia reaches 700,000 articles
- September 21: French Wikipedia reaches 1,000,000 articles
- October 6: Tamil Wikipedia reaches 25,000 articles
- October 25: Tagalog Wikipedia reaches 40,000 articles
- November 14: Belarusian Wikipedia reaches 25,000 articles
- November 30: Estonian Wikipedia reaches 80,000 articles
- December 15: Korean Wikipedia reaches 150,000 articles
- December 21: Ukrainian Wikipedia reaches 250,000 articles
- January 9: Vietnamese Wikipedia reaches 150,000 articles, Malay Wikipedia reaches 100,000 articles.
- January 12: Spanish Wikipedia reaches 700,000 articles.
- January 15: Kannada Wikipedia reaches 10,000 articles, Tagalog Wikipedia reaches 50,000 articles.
- Clauson et al - Scope, completeness, and accuracy of drug information in Wikipedia (Ann Pharmacother, volume 42, issue 12, page 1814)
- S. Robert Lichter, Ph.D. - Are chemicals killing us? (Statistical Assessment Service, May 21, 2009)
- Rajagopalan et al, Accuracy of cancer information on the Internet: A comparison of a Wiki with a professionally maintained database. (Journal of Clinical Oncology 28:7s, 2010)