The Global Energy and Water Cycle Experiment (GEWEX), of the World Climate Research Programme (WCRP) was initiated in 1988 and has coordinated the activities of the Continental Scale Experiments (CSEs), which are now known as Regional Hydroclimate Projects (RHPs) and other land surface research through the GEWEX Hydrometeorology Panel (GHP). The GHP was established in 1995 to contribute to the WCRP objective of "developing the fundamental scientific understanding of the physical climate system and climate processes needed to determine to what extent climate can be predicted and the extent of man's influence on climate". More specifically, the GHP contributed to the GEWEX objectives such as "determining the hydrological cycle and energy fluxes, modelling the global hydrological cycle and its impact, developing a capability to predict variations in global and regional hydrological processes and fostering the development of observing techniques, data management and assimilation systems". GHP activities included diagnosis, simulation and prediction of regional water balances by various process and modelling studies aimed at understanding and predicting the variability of the global water cycle, with anemphasis on regional coupled land-atmosphere processes. GHP efforts were central to providing a scientific basis for assessing critical science issues such as the consequences of climate change for the intensification of the global hydrological cycle and its potential impactson regional water resources. This paper discusses the more relevant scientific issues relating to hydrology addressed by the GHP in collaboration with the international science community, in particular the IAHS Predictions in Ungauged Basins (PUB) initiative. GHP activities have now been formally merged with the Coordinated Enhanced Observation Period (CEOP) I and II activities to form a new body, called the Coordinated Energy and water-cycle Observations Project (CEOP), which will continue to foster large-scale hydroclimate research. Within GHP and now within CEOP the Water Resources Applications Project (WRAP) was established in 2000 to facilitate the broader use of GEWEX products in water resource applications and initially promoted dialogue between the GEWEX community and the water resources community. With members from each of the RHPs, IAHS, UNESCO programmes, and the World Meteorological Organization (WMO), this group provided a wide range of expertise related to water management. WRAP relied on the development of physically-based hydrology and "application" or decision support models, and the coupling of these models with regional climate models. Application studies require a capability to downscale largearea (model grid square) precipitation forecasts and observed averages, statistical analyses of the relationships between SST anomaliesandseasonal streamflow, and analysis of the value and utility of seasonal forecasts in water management decisions. WRAP is now evolving toward a Hydrologic Applications Project (HAP) which was defined in October 2006 together with the "Roadmap" for the remainder ofthe Second Phase of GEWEX (2006-2012). This paper summarises the achievements to date of GEWEX pertaining to hydrology and gives an indicationof the planned hydrological focus of the project over the next five years. It also discusses the more relevant scientific issues relating to IAHS hydrological issues to be addressed by CEOP, including the IAHS Predictions in Ungauged Basins (PUB) initiative.