The Aare or Aar is a tributary of the High Rhine and the longest river that both rises and ends entirely within Switzerland.
Its total length from its source to its junction with the Rhine comprises about 295 kilometres, during which distance it descends 1,565 m, draining an area of 17,779 km², almost entirely within Switzerland, and accounting for close to half the area of the country, including all of Central Switzerland.
There are more than 40 hydroelectric plants along the course of the Aare.
The river's name dates to at least the La Tène period, and it is attested as Nantaror "Aare valley" in the Berne zinc tablet.
The name was Latinized as Arula/Arola/Araris.