Availability and quality of groundwater resources is seriously affected by urbanization in many large cities. This leads to significant social, environmental, economical, and political implications. Any urban hydrology study requires a detailed analysis of water fluxes flowing in and out of the system, including their magnitude and relative importance. Analyzing the quality of the resources is another key issue. In any real case study one has to face at least three aspects: (1) process identification, (2) aquifer characterization, and (3) water resources management. This methodology has been applied to the city of Barcelona. The outcome is a groundwater model that allows defining, characterizing, and quantifying the potential risk of aquifers, both on urban structures, and upon population. In this sense, it may become a management tool. This is an important step towards convincing city managers to consider groundwater as one of the topics to be taken into account in city planning.