Volcanic gases provide the explosive power that drive volcanic eruptions. Monitoring the emission rates and chemical composition of volcanic gases can offer important clues to the inner workings of volcanoes. Gas emissions from the rugged Cascade volcanoes are sampled using airborne and ground-based techniques. In general, the gases emitted from Cascade volcanoes are from boiling hydrothermal fluids at depth and are rich in water vapor, carbon dioxide, and hydrogen sulfide. However, detailed chemical analyses show that the ultimate source of the heat and gases emitted from the major Cascade peaks is magma degassing at depth.