Seine-et-Oise was a département of France encompassing the western, northern, and southern parts of the metropolitan area of Paris. Its préfecture (capital) was Versailles and its official number was 78. Seine-et-Oise was abolished in 1968 as part of the reorganization of the départements of the Paris metropolitan area.
At its dissolution in 1968, Seine-et-Oise consisted of 688 suburban and rural communes completely surrounding the Seine département although it was at its narrowest just east of Seine between that département and the Seine-et-Marne département which still exists today. It had an area of 5,658 km² (2,184 sq. miles). The division of Seine-et-Oise into arrondissements changed many times. At its disappearance it had ten arrondissements: Argenteuil, Étampes, Mantes, Montmorency, Palaiseau, Pontoise, Le Raincy, Rambouillet, Saint-Germain-en-Laye, and Versailles.
At the first French census in 1801, Seine-et-Oise had 421,535 inhabitants. With the growth of the Paris suburbs, the population of Seine-et-Oise increased markedly, and by 1968 it had reached 2,943,350 inhabitants. It was judged that Seine-et-Oise, along with several other departments in the Paris conurbation, was now too large and ungovernable. On 1 January 1968 it was split into (essentially) three smaller départements: Yvelines, Val-d'Oise, and Essonne. A small part of Seine-et-Oise was also merged with parts of the Seine département (also disbanded on the same date) to create the three new départements of Hauts-de-Seine, Val-de-Marne, and Seine-Saint-Denis.
In detail, the splitting up of the Seine-et-Oise département was carried out like this: 262 communes in the central part of the département became the Yvelines département, with Versailles as the préfecture. The official number 78, which was used for Seine-et-Oise, was given to the new Yvelines département, which is the largest amount of the former Seine-et-Oise (40% of the area of Seine-et-Oise). There were 198 communes in the south of Seine-et-Oise (32% of the area of Seine-et-Oise) that became part of the Essonne département, and the official number 91 was assigned to this département, a number that had been previously used for the Alger département in French Algeria). There were 185 communes in the north of Seine-et-Oise (22% of the area of Seine-et-Oise) that became part of the Val-d'Oise département, and the official number 95 was assigned to this département, a number that had never been used.
Of the remaining 6% of Seine-et-Oise, 18 communes were grouped with 29 communes of the Seine département to create the Val-de-Marne département, 16 communes of Seine-et-Oise were grouped with 24 communes of the Seine département to create the Seine-Saint-Denis département, and the last 9 communes of Seine-et-Oise were grouped with 27 communes of the Seine département to create the Hauts-de-Seine département.
Thus, Yvelines, Val-d'Oise, and Essonne are altogether smaller than the former Seine-et-Oise département (5,658 km² for the Seine-et-Oise département vs. 5,334 km² for the three départements).
The three départements of Yvelines, Essonne, and Val-d'Oise, plus the Seine-et-Marne département, are altogether known in France as the grande couronne (i.e. "large ring", as opposed to the "small ring" of the suburbs closer to Paris).
At the 1999 French census, the population of the former Seine-et-Oise was 4,554,426 inhabitants, its highest figure ever, as people relocate more and more from the center to the distant suburbs of the metropolitan area of Paris. Of the new départements created in 1968 out of Seine-et-Oise, Yvelines was the most populous in 1999 with 1,354,304 residents. Seine-Saint-Denis and Hauts-de-Seine are more populous than Yvelines, but only a small part of their territory is made up of the former Seine-et-Oise.