Campi Flegrei caldera (CFc, Southern Italy) is surely the most studied caldera in the world. Apart from the long record of volcanic eruptions, three episodes of major uplift have occurred at CFc since 1950. After the 1969–72 and 1982–84 uplifts, two models resulted from the inversion of recorded gravity and/or deformation data: (a) One suggested that uplift at CFc is caused by the movement of magma beneath the surface. As magma accumulates beneath the ground, it exerts pressure on the overlying rock layers, causing them to deform and slowly rise upward. When the pressure is relieved, a limited ground subsidence due to magma relaxation occurs. (b)Another proposed model suggested that uplift is driven by geothermal processes rather than magma movement. The movement of hot water and steam through the rock layers beneath the ground causes them to expand and contract, leading to the observed ground uplift and the following subsidence. These two models were somehow in competition, but the nearly 20 years of subsidence following the 1982–84 uplift played in favor of model (b). Nevertheless, a new uplift started around 2004–05, which renewed this competition despite the huge amount of data collected from an upgraded monitoring network, which made the study of gas emissions pivotal. Although fluid geochemistry puts first-order constraints on the nature of the source, different conceptual models were derived, all searching for consistency with deformation-based models and agreeing on a common magmatic-hydrothermal engine responsible for degassing. However, these conceptual models yield different and even opposite conclusions when comparing the 1982–84 and the ongoing (since 2004) unrest episodes. Differences were about i) the depth of magma body(es), ii) the emplacement of a shallow (3–4 km deep) magma, and iii) the fate of magma(s), which degassing can be modeled via either decompression or crystallization (second boiling). However, recent studies reveal convergence points of such contrasting geochemical interpretations. We discuss how this interesting evolution is a consequence of a better appraisal of CO2 in the system. Furthermore, CO2 has a strong effect on the thermodynamic properties of the hydrothermal fluid and the geomechanical ground response.