Boulogne-Billancourt (French pronunciation: [bulɔɲ bijɑ̃kuʁ]; often colloquially called simply Boulogne, until 1924 Boulogne-sur-Seine, [bulɔɲ syʁ sɛn]) is a wealthy and prestigious commune in the western suburbs of Paris, France, located 8.2 km (5 mi) from the centre of Paris. It is a subprefecture of the Hauts-de-Seine department and thus the seat of the larger arrondissement of Boulogne-Billancourt. It is also part of the Métropole du Grand Paris. Boulogne-Billancourt includes one island in the Seine: Île Seguin. With a population of 121,334 as of 2018, it is the most populous commune in Hauts-de-Seine and most populous suburb of Paris, as well as one of the most densely populated municipalities in Europe.[citation needed]
 Boulogne-Billancourt is one of the wealthiest regions in the Parisian area and in France. Formerly an important industrial site, it has successfully reconverted into business services and is now home to major communication companies headquartered in the Val de Seine business district.
 The original name of the commune was Boulogne-sur-Seine (meaning "Boulogne upon Seine").
 Before the 14th century, Boulogne was a small village called Menuls-lès-Saint-Cloud (meaning "Menuls near Saint-Cloud"). In the beginning of the 14th century, King Philip IV of France ordered the building in Menuls-lès-Saint-Cloud of a church dedicated to the virgin of the sanctuary of Boulogne-sur-Mer, then a famous pilgrimage center in northern France. The church, meant to become a pilgrimage centre closer to Paris than the distant city of Boulogne-sur-Mer, was named Notre-Dame de Boulogne la Petite ("Our Lady of Boulogne the Minor"). Gradually, the village of Menuls-lès-Saint-Cloud became known as Boulogne-la-Petite, and later as Boulogne-sur-Seine.
 In 1924, Boulogne-sur-Seine was officially renamed Boulogne-Billancourt to reflect the development of the industrial neighbourhood of Billancourt annexed in 1860.
 As for the name Billancourt, it was recorded for the first time in 1150 as Bullencort, sometimes also spelled Bollencort. It comes from Medieval Latin cortem, accusative of cors, meaning "enclosure", "estate", suffixed to the Germanic patronym Buolo (meaning "friend, brother, kinsman"), thus having the meaning of "estate of Buolo".
 On 1 January 1860, the City of Paris was enlarged by annexing neighbouring communes. On that occasion, the communes of Auteuil and Passy were disbanded and divided between Boulogne-Billancourt (then called Boulogne-sur-Seine) and the city of Paris. Boulogne-sur-Seine received a small part of the territory of Passy, and about half of the territory of Auteuil (including the area of Billancourt, which belonged to the disbanded commune of Auteuil).
 Some of the shooting events of the 1900 Summer Olympics took place in Boulogne-Billancourt.
 In 1929, the Bois de Boulogne, which was hitherto divided between the communes of Boulogne-Billancourt and Neuilly-sur-Seine, was annexed in its entirety by the city of Paris. On that occasion, Boulogne-Billancourt, to which most of the Bois de Boulogne belonged, lost about half of its territory.  Since then, Boulogne-Billancourt has been surrounded to the west, south and east by the Seine and to the north and north-east by the 16th arrondissement of Paris.
 Boulogne-Billancourt is known for being the birthplace of three major French industries. It was the location, in 1906 for the very first aircraft factory, that of Appareils d'Aviation Les Frères Voisin, which was then followed by those of many other aviation pioneers, and the tradition continues with several aviation related companies still operating in the area.
 The automobile industry had a large presence with Renault on Île Seguin, as well as Salmson building both cars and aircraft engines. Finally, the French film industry started here and, from 1922 to 1992 it was the home of the Billancourt Studios, and since becoming a major location for French film production. It was used as the setting of the TV show Code Lyoko.
 With the city of Sèvres, Boulogne-Billancourt is part of the communauté d'agglomération Val de Seine.
 Boulogne-Billancourt is served by two stations on Paris Métro Line 10: Boulogne–Jean Jaurès and Boulogne–Pont de Saint-Cloud. It is also served by three stations on Paris Métro Line 9: Marcel Sembat, Billancourt and Pont de Sèvres.
 Boulogne-Billancourt is represented by two constituencies and therefore two Members of Parliament.
 Boulogne-Billancourt hosts the global headquarters of several multinational companies, including:
 Prior to 2000 Schneider Electric's head office was in Boulogne-Billancourt.
 The public collèges (middle schools) in the commune include Jacqueline-Auriol, Bartholdi, Paul-Landowski and Jean-Renoir. The public high schools are the Lycée Jacques-Prévert and the Lycée Polyvalent Étienne-Jules-Marey. Prior to the September 1968 opening of Prévert, the first high school/sixth-form in Boulogne, an annex of Lycée La Fontaine served the city.
 The private school Groupe Scolaire Maïmonide Rambam covers maternelle through lycée. There is also the private high school Notre-Dame. The latter's performance and ranking in Boulogne-Billancourt are given by its success of baccalaureate rate in different series. According to the ranking of L'Express in 2015, the national rank of Notre-Dame de Boulogne was 170 out of 2301 and 7 out of 52 at department level. The private schools Dupanloup and Saint-Joseph-du-Parchamp serve maternelle through collège. Private maternelle and élémentaire schools include Saint-Alexandre and Saint-François d’Assise. Jardin de Solférino and La Maison de l'Enfant are private maternelles.
 The Association Eveil Japon (エベイユ学園 Ebeiyu Gakuen), a supplementary Japanese education programme, is located in Boulogne-Billancourt. A campus of the École supérieure des sciences commerciales d'Angers is also located in the city.
 Boulogne-Billancourt was the birthplace of:
 Boulogne-Billancourt is twinned with: