Ain (æ̃, ɛ̃; En) is a French department in the Auvergne-Rhône-Alpes region, Eastern France. Named after the Ain river, it is bordered by the Saône and Rhône rivers. Ain is located on the country's eastern edge, on the Swiss border, where it neighbours the cantons of Geneva and Vaud. In 2019, it had a population of 652,432.
Ain is composed of four geographically different areas (Bresse, Dombes, Bugey and Pays de Gex), each of which contribute to the diverse and dynamic economic development of the department. In Bresse, agriculture and agro-industry are dominated by the cultivation of cereals, cattle breeding, milk and cheese production as well as poultry farming. In Dombes, pisciculture assumes greater importance, as does winemaking in Bugey.
Ain's prefecture is Bourg-en-Bresse. It is bordered by Jura to the north; Saône-et-Loire to the northwest; Rhône and the Lyon Metropolis to the southwest; Isère to the south; Savoie, Haute-Savoie and Switzerland to the east.
In the alphabetical ordering of French departments, used for postal and demographic purposes amongst others, Ain comes first and is thus assigned the number 01 as its department number.
The first inhabitants settled in the territory of today's Ain about 15000 BC. The menhir of fr in Simandre-sur-Suran dates from the mid-Neolithic era, in the fourth or third millennium BC. The late-second century BC Calendar of Coligny bears the oldest surviving Gaulish inscription.
In 58 BC, Julius Caesar's military action against the Helvetians, advancing through Gaul over the territory of today's Ain, marked the beginning of the Gallic Wars.
Under the Merovingians, the four historic regions of the modern department belonged to the Kingdom of Burgundy. At the start of the 6th century AD, the diocese of Belley (Bellicum) was created as the first bishopric in the region. Abbeys of the order of Saint Benedict were established in the valleys.
In 843, the Treaty of Verdun assigned the territories that comprised Ain to the kingdom of Lothar I (Lotharingia).