The RATP bus network covers the entire territory of the city of Paris and the vast majority of its near suburbs. Operated by the Régie Autonome des Transports Parisiens (RATP), this constitutes a dense bus network complementary to other public transport networks, all organized and financed by Île-de-France Mobilités.

Other suburban bus lines are managed by private operators grouped in a consortium known as Optile (Organisation professionnelle des transports d'Île-de-France), an association of 80 private bus operators holding exclusive rights on their lines. There are approximately 9500 buses serving public transportation across the Paris region, all operators included.

RATP operates:

RATP bus lines operating mainly in the city proper of Paris, are named with a two-digit code number called "indice". Bus lines operated mainly in the suburbs are named with a three-digit number code.

RATP uses the numbers 20 to 96 for lines operated mainly in the city proper (1 to 19 are unused to avoid confusion with Metro lines); along with  PC  bus line (line 100) which runs along the Boulevard des Maréchaux section that is not yet covered by (Tramway lines 3a & 3b) . All this according to an organized scheme used since public transportation reorganization post-WWII.

The first digit represents the sector in Paris where the line's starting point is located:

The digit number represents the outermost arrondissement the line finishes (or crosses Paris' borders) in:

0 and 1 are used as "wildcards" for lines whose rightful indice is already taken, or for lines ending inside of Paris (40 or 91 for example).