Boulogne-Billancourt (; often colloquially called simply Boulogne) is a commune in the western suburbs of Paris, France. It is located from the centre of Paris. Boulogne-Billancourt is a subprefecture of the Hauts-de-Seine department and the seat of the Arrondissement of Boulogne-Billancourt.
With an average household income of € 28.742 Boulogne-Billancourt is the wealthiest among French communes of more than 100.000 inhabitants, followed by Paris.
Boulogne-Billancourt is the most populous suburb of Paris and one of the most densely populated municipalities in Europe. 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.
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).
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.
Boulogne-Billancourt is famous for being the birthplace of three major French industries: cinema, automobile with Renault, and aircraft. It is also famous for being the setting of the TV show "Code Lyoko."