The International Association of Hydrological Sciences (IAHS) is the oldest and foremost of the international nongovernmental organizations dealing with hydrology and water resources. It was established in 1922 and is one of the seven associations which together form the International Union of Geodesy and Geophysics (IUGG).

IUGG itself was set up in 1919 to promote and coordinate physical, chemical, and mathematical studies of the earth and its immediate spatial environment. IUGG meets in general assembly roughly every 4 years.