The commune consists of only 240 hectares but it is the capital of Arpajonnais Country which extends from Breuillet in the east to Brétigny-sur-Orge in the west and from Leuville-sur-Orge in the north to Cheptainville in the south.

The city centre is in the Orge valley at an altitude of approximately 47 metres but the territory extending north and south rises rapidly to the Avrainville plateau in the south, La Norville in the east, and the start of the Linas hill to the north which means it is necessary to climb to exit the commune.

Due to the low height of the town relative to the surroundings there are many television receiving antennae which are sometimes ten metres above the roofs.

The territory lies on the boundary of the limestone geological area of the Paris basin and the sand of the Beauce[9] with, in the extreme north, the remains of sandstone quarries.

Arpajon is located in Île-de-France and enjoys an attenuated oceanic climate characterized by cold winters, warm summers, and rainfall evenly distributed over the year.

Located near the Paris city centre and without the presence of large areas of culture, however, sunshine hours for the commune amounted to 1,798 annually as it is throughout the north of the department.

The castrum became Chastres around the year 250 then Châtres (sometimes Châtres-sous-Montlhery) in the 17th century with the reform of spelling and the appearance of the circumflex in place of the silent "s".

At the time of Roman Gaul there was a castrum was installed at the intersection of the road from Lutèce to Cenabum and the river Orge in the valley that was the territory of the Parisii tribe.

[13] Two megalithic monuments remain: one in the Library Park and the other near the Rémarde river an inscription in Gallic was found in 1947 and is kept in the Municipal Museum of Saint-Germain-en-Laye.

In 1006 the rebuilding of the church and steeple was entrusted to the Benedictine monks of the Abbey of Saint-Maur by Renaud de Vendôme, bishop of Paris and it was consecrated to Pope Clement I.

In July 1470 King Louis XI authorised two fair days at Chastres for his counselor and chamberlain Jean du Graville by letters patent.

[14] Louis Malet de Graville built a market hall at the crossroads of the Paris to Étampes and Dourdan to Corbeil roads.

In 1510 the monks, through the generosity of the Graville and Montagu families undertook major renovations of the church and in 1542 a sub-delegation of Chastres was attached to the Generality of Paris.

In 1733 he knocked down the old city gate in the north which was too narrow for many carts and instead erected two Pilasters which are the current Porte de Paris.

[17] Philippe de Noailles was lord of Arpajon and he was long in great favour at court with his wife being first lady of honour to Marie Antoinette, nicknamed by her Madame Etiquette.

In the second half of the 19th century the Boulevard de la Gare was opened with the inauguration of the railway station in 1865 and the construction of holiday villas.

On 6 July 1924 on the long, straight Route nationale 20 between Arpajon and Torfou, René Thomas set a land speed record of 230.47 km/h (143.21 mph) in a Delage called La Torpille (The Torpedo) followed, on 12 July 1924, by Ernest A.D. Eldridge who increased the record to 234.98 km/h (146.01 mph) in a Fiat Special called Mephistopheles II.

Arpajon is the chief town of the canton represented by the general counsel Christian Béraud (PS), mayor of the commune.

[24] The commune has a communal social welfare centre and, in 2017, had 942 HLM housing units or 19.6% of all main residences.

List of Successive Mayors[28] Politics in Arpajon in recent years have been characterized by ambivalent results which frequently follow national trends.

Similarly, Arpajon voters followed the national movement during the European elections in 2004 where the left was ahead of the right but at the referendum on the Treaty establishing a Constitution for Europe the "no" won.

The commune has a hospital with a capacity of 302 beds, offering services such as: Surgery, Obstetrics, Pediatrics, Gastroenterology, Diabetology, Cardiology, Pneumology, and Radiography.

The private hospital of Paris-Essonne, formerly the Charmilles Clinic, completes the offering with 60 beds, a surgery, and a specialization in pathologies of the face: ophthalmology, oto-rhino-laryngology, and dentistry.

In 1999 10.8% of Arpajon people were foreigners and 14% of households consisted of single-parent families or 2 percent more for each category compared to the departmental figures.

Percentage Distribution of Age Groups in Arpajon and Essonne Department in 2017 Source: INSEE[55][56] As the capital of Arpajonnais and a historic land of market gardening, the commune has long been a place of trade and known for its fair since the 13th century.

[57] Two industrial areas include most of the other businesses: the Belles-Vues and the Butte-aux-Grès,[58] both located in the north of the commune along Route nationale 20.

The commune has several religious buildings and structures that are registered as historical monuments: Although 80% of the communal area is urban there are still some parks and public gardens.

[94] The north-west of the commune still has cultural areas recalling the agricultural character of the town that specialised in market gardening.

There is a legend that the name Bineau of a carnivalesque character recalls a steward who was granted a Droit du seigneur on the ladies of the village and was burned alive as punishment.

[100] Honoré de Balzac located part of his novel Un début dans la vie (A start in life) in Arpajon: "As for Mr. Serisy, he did not emigrate during the Revolution, he went to his Serizy land at Arpajon, where the respect that he had from his father saved him from all evil"[101] The character of a "Countess of Arpajon" appears in the work À la recherche du temps perdu (In Search of Lost Time) by Marcel Proust in the volumes Sodome et Gomorrhe (Sodom and Gomorrah) and Le Temps retrouvé (Time Regained).