Predictions in Ungauged Basins (PUB) mainly focus on surface water, but there are much higher prediction uncertainties inherent in groundwater. Hanoi depends entirely on groundwater for its domestic water supply. However, the characteristics of the entire Hanoi aquifer system remain poorly understood due to the lack of available data. Recently, we were nominated to construct a hydrogeological database. Using the valuable data contained in this database, this paper comprehensively analyzed the best number of 240 boreholes including well logs and their hydrogeological parameters obtained from pumping tests for the first time in order to identify the entire Hanoi aquifer system and characterize the hydrogeological conditions for potential groundwater resources. Great efforts have been made to establish and analyze the hydrogeological maps, cross sections and the isopach maps of main aquifers' thickness and transmissibility. As for the results, we found that groundwater mainly exists in the topmost Holocene unconfined aquifer and the shallow Pleistocene confined aquifer (PCA), while cleft and karst water exists in the Neogene water bearing layer and the Mesozoic fractured zones. These aquifers are adequately, quantitatively characterized and evaluated from the viewpoint of potential groundwater resources. We found the PCA serving as the highest groundwater potential and the most important aquifer for the water supply. The findings are indispensable for further groundwater analyses contributing to ensuring the sustainable groundwater development not only in Hanoi but also in poorly gauged or ungauged neighboring basins. Copyright © 2012 John Wiley & Sons, Ltd.