Setagaya has the largest population and second largest area (after Ōta) of Tokyo's 23 special wards. As of May 1, 2011, the ward has an estimated population of 837,185 and a population density of 14,414.34 persons per km² with the total area of 58.08 km².
Residential population is among the highest in Tokyo as there are many residential neighbourhoods within Setagaya. Setagaya is served by various rail services providing frequent 2 to 3 minutes headway rush hour services to the busiest train terminals of Shinjuku and Shibuya as well as through service trains which continue travelling on to the Tokyo Metro lines providing direct access to the central commercial and business districts. Most rail lines run parallel from east to west and there are no north and south rail services within Setagaya, except for Setagaya Line light rail.
The ward is divided into five districts. These are Setagaya, Kitazawa, Tamagawa, Kinuta and Karasuyama. The main ward office and municipal assembly (city hall) is located in Setagaya District, but other districts also have its own branch ward offices as a part of the administrative structure. Each branch offices provide almost identical services as the main office, but does not provide the services related to municipal assembly.
Most of the land is in the Musashino Tableland. The parts along the Tama River to the south are comparatively low-lying.
The special ward of Setagaya was founded on March 15, 1947.
During the Edo period, 42 villages occupied the area. With the abolition of the han system in 1871, the central and eastern portions became part of Tokyo Prefecture while the rest became part of Kanagawa Prefecture; in 1893, some areas were transferred to Tokyo Prefecture. With the establishment of Setagaya Ward (an ordinary ward) in the old Tokyo City in 1932, and further consolidation in 1936, Setagaya took its present boundaries.
^"Facilities and Location." British School in Tokyo. Retrieved on March 8, 2015. "BST Shibuya Campus – (Nursery to Year 3) Address: 1-21-18 Shibuya Shibuya-ku Tokyo 150-0002" (Map) and "BST Showa Campus – (Year 4 to Year 13) Address: Showa Women’s University 5th Bldg. 1-7-57 Taishido Setagaya-ku Tokyo 154-8533"