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We investigated the Campi Flegrei caldera using a quantitative approach to retrieve the spatial and temporal variations of the stress field. For this aim we applied a joint inversion of geodetic and seismological data to a dataset of 1,100 optical levelling measurements and 222 focal mechanisms, recorded during the bradyseismic crisis of 1982-1984. The inversion of the geodetic dataset alone, shows that the observed ground deformation is compatible with a source consisting of a planar crack, located at the centre of the caldera at a depth of about 2.56 km and a size of about 4 9 4 km. Inversion of focal mechanisms using both analytical and graphical approaches, has shown that the key features of the stress field in the area are: a nearly subvertical r 1 and a sub-horizontal, roughly NNE-SSW trending r 3. Unfortunately, the modelling of the stress fields based only upon the retrieved ground deformation source is not able to fully account for the stress pattern delineated by focal mechanism inversion. The introduction of an additional regional background field has been necessary. This field has been determined by minimizing the difference between observed slip vectors for each focal mechanism and the theoretical maximum shear stress deriving from both the volcanic (time-varying) and the regional (constant) field. The latter is responsible for a weak NNE-SSW extension, which is consistent with the field determined for the nearby Mt. Vesuvius volcano. The proposed approach accurately models observations and provides interesting hints to better understand the dynamics of the volcanic unrest and seismogenic processes at Campi Flegrei caldera. This procedure could be applied to other volcanoes experiencing active ground deformation and seismicity.