I study the structure beneath volcanoes, describe unexpected effects of the shaking from an earthquake on volcanoes, and suggest a model for a magma system. The conceivable configuration of magma systems beneath Galeras (Colombia) is studied with b- value mapping. I obtain models for earthquake- volcanoes interactions by studying drops in seismicity at Mt. Wrangell (Alaska), after the 2002 Denali Fault Earthquake. A model for the magmatic system beneath Mt. Veniaminof is proposed by modeling velocities and relocating shocks, and by studying the temporal changes of frequencies and attenuation. The internal structure of volcanoes varies and has localized zones of high b-values. Seismicity at Mt. Veniaminof suggests that earthquakes result from locally-induced stresses and that some events represent the response of a shallow hydrothermal system to heat input from below. The study of drops in seismicity at Wrangell indicates that both dynamic shaking and static stress changes from regional shocks can affect the local seismicity for extended periods. This work was undertaken because much research in volcano seismology is needed to help in hazard assessment.