Volcanoes are one of the most destructing phenomena in the world. Unfortunately, they are also the most difficult study cases since the destructive phenomena on the Earth surface derive from magmatic bodies in the underground that are not accessible to observation. From this the need to perform simulations of the underground magmatic systems, and to relate the underground magmatic processes to the signals that are observed on the Earth surface (ground displacement and gravity anomalies). The whole complexities of a volcano are shown, and the 1st order model used to simulate the deep magmatic bodies is illustrated. The examples of Mn.t Etna and Campi Flegrei (Naples) are shown.