Abstract. Water resources cannot be managed, unless we know where they are, in what quantity and quality, and how variable they are likely to be in the foreseeable future. Data from hydrological networks are used by public and private sectors for a variety of different applications. This paper discusses the value proposition behind the collection, analysis and use of hydrological data in support of these applications. The need for hydrological data and the requirements for the data are outlined, and information is provided on topics such as status of networks and data access and sharing. This paper outlines elements of the contribution by the World Meteorological Organization (WMO) to hydrological data collection and covers aspects related to quality management in the collection of hydrological data, especially regarding streamflow gauging, network design and capacity building for services delivery. It should be noted that the applications which make use of hydrological data may also be significantly impacted by climate change.