Volcanic processes operate over a wide range of time scales that require a variety of instruments and techniques for their study. Short-period seismology typically covers the band 0.1–1 s, while broadband seismology can extend that band to 0.1–100 s. Borehole strainmeters may be used to cover the very wide band from 0.1 s to 100 days and Global Positioning System (GPS) surveys are useful to track deformation over time scales ranging from days to decades. Arrays of three-component broadband seismographs coupled with arrays of borehole strainmeters are required to monitor the dynamics of magmatic and hydrothermal fluids prior to and during eruptions. Inversions of broadband data may be performed to image the forces operating at a source and infer the fluid pathway geometry and mass transport balance in a volcano. The accuracy of such inversions depends on the degree of resolution achieved for the volcanic structure. High-resolution tomography based on iterative inversions of seismic travel-time data can image three-dimensional structures at a scale of a few hundred meters provided adequate local short-period earthquake data are available. Hence, forces in a volcano are potentially resolvable for periods longer than a few seconds. Studies of long-period events and tremor at periods ≤ 1s offer constraints on pressure fluctuations resulting from unsteady mass transport, and observations of volcano-tectonic activity tell us about hydraulic fracturing processes, as well as brittle response of the rock matrix to stress changes induced by rapid injection and/or withdrawal of fluids. Short-period networks are routinely used to monitor, locate, and assess the source properties of volcano-tectonic earthquakes and long-period events, while small-aperture seismic antennas are required to track tremor and separate source, path, and site effects in tremor wavefields. This chapter reviews quantitative methods used in the interpretation of source processes and structures in volcanoes and discusses some of the major challenges faced by volcano seismologists in their quest to understand eruptive behavior.