The study area covers the Cimini and Vico volcanic complexes in the province of Viterbo (Northern Lazio Region, Italy), where a large-scale exploitation of the water resources co-exists with fragile environmental conditions. The Vico caldera and the lake of the same name form the central nucleus of a Nature Reserve and are an important source of drinkable water for a number of municipalities. The main aim of the study was to improve our knowledge of the hydrogeological setting of the system in order to quantify the groundwater resource and to examine the interaction between the groundwater and Lake Vico in their specific geological framework. The hydrogeological investigations included: i) reconstruction of local hydrostratigraphy; ii) collection of data from wells and springs; iii) flow measurement of streams; iv) development of some pumping tests; v) analysis of climatic data. The volcanic aquifer consists primarily of Pleistocene pyroclastic deposits (tuffs, ignimbrites, ashes, pumice and scoriae) and various lava flows and domes. Its base and part of its SW and NE boundaries are well defined by low-permeability rocks, including sandy-clayey deposits (Pliocene-Pleistocene) and marly-calcareous flysch (Upper Cretaceous-Oligocene). The remaining boundaries are formed by highly permeable Quaternary alluvial and sandy-conglomeratic deposits. The groundwater flow occurs mainly through the unconfined basal volcanic aquifer and to a lesser extent through various perched aquifers. The basal volcanic aquifer has a centrifugal radial flow network; the groundwater discharges primarily into streams (not less than 1.4 m <sup>3</sup> /s) and in part into small springs (less than 0.5 m <sup>3</sup> /s). The volcanic aquifer also feeds the aquifer of the surrounding plain, especially where the Quaternary sandy-conglomeratic deposits flank the volcanic hills (about 2 m <sup>3</sup> /s). The pumping test data were correlated with the equipotential surface of the unconfined basal aquifer to define diverse transmissivity zones. The resulting wide range of transmissivity values obtained (between 10 (super -6) and 10 (super -2) m <sup>2</sup> /s) are in agreement with the heterogeneity of the volcanic products and with the variability of the saturated thickness. An average annual evaluation of groundwater resources points to a volume between 162 and 227X10 <sup>6</sup> m <sup>3</sup> /year. Taking the area of the volcanic system into account, the mean yield of the aquifer amounts to between 5.7 and 8.0 l/s per km <sup>2</sup> , consistent with other Tyrrhenian volcanic areas. The analysis of the historical hydro-meteorological data set indicates that precipitation follows a negative trend, i.e. there is a progressive decline of groundwater resources. In this hydrogeological environment, Lake Vico is closely conditioned by the groundwater, being fed by the basal aquifer on its northern side (about 6% of the total inflow) and drained from the other sides (between 26 and 37% of the total outflow). According to the mean water budget of the lake, in addition to the amounts of water taken from the lake, the surface-groundwater exchange has a considerable effect on the reservoir resources. These factors can influence heavily the level of the lake, artificially regulated by a gated outlet which is not always managed according to a rational strategy of conjunctive use of water resources.