Highlighting the city of Lisbon as a case study, this chapter focuses on the pioneering role of the earliest geologists who introduced Geology, its concepts and its methods as a key tool in the search for drinkable water. This paradigmatic change occurred in the 1850s, due to the work of Carlos Ribeiro, Chief Engineer of the Department of Mines in the Ministry of Public Works, and the first director of the Kingdom’s Geological Commission, founded in 1857. Invited to submit his advice about the plans for the Lisbon water supply, Ribeiro was convinced that only detailed geological fieldwork could provide the knowledge for accurately predicting the occurrence of groundwater. For this purpose, he carried out detailed lithological and structural studies on the northern and eastern ranges of the city, subsequently becoming able both to redefine and map the main stratigraphic units and to characterize those with better quality aquifer properties. The knowledge and experience generated by this pioneering work was followed by several contributions by Nery Delgado, his right-hand man, and by their colleague, renowned geologist Paul Choffat. Taken together, these works may be considered as the true precursors of modern hydrogeological studies in Portugal.