The International Association of Hydrological Sciences (IAHS) is one of the seven international associations comprising the International Union of Geodesy and Geophysics (IUGG). Each Association has its own domain, but the boundaries between them are artificial. The Earth is undisciplined, and it does not recognize our academic subjects. In the US Academy Report “Solid-Earth Sciences and Society”, published in 1993, a number of broad objectives and major research areas were set up in a matrix which provided a research framework. A remarkable number of high-priority research opportunities in the “solid-Earth Sciences” are concerned with water, the subject of IAHS. Water is a critical material for society and indeed in nearly every topic in Earth science research. Our priorities for the next century must be based on the beautiful science which arouse our curiosity, but we must also give weight to societal problems. Examples of burning recent problems can be found in many of the world's megacities that are situated in locations threatened by earthquakes, volcanic eruptions, landslides, floods, and rising sea level.