With the emergence of portable broadband seismic instrumentation, availability of digital networks with wide dynamic range, and development of new powerful analysis techniques made possible by greatly increased computer capacity, volcano seismology has now reached a mature stage where insights are rapidly being gained on the role played by magmatic and hydrothermal fluids in the generation of seismic waves. Volcanoes produce a wide variety of signals originating in the transport of magma and related hydrothermal fluids and their interaction with solid rock. Typical signals include (1) brittle failure earthquakes that reflect the response of the rock to stress changes induced by magma movement; (2) pressure oscillations accompanying the dynamics of liquids and gases in conduits and cracks; and (3) magma fracturing and fragmentation. Oscillatory behaviors within magmatic and hydrothermal systems are the norm and are the expressions of the complex rheologies of these fluids and nonlinear characteristics of associated processes underlying the release of thermo-chemical and gravitational energy from volcanic fluids along their ascent path. The interpretation of these signals and quantification of their source mechanisms form the core of modern volcano seismology. The accuracy to which the forces operating at the source can be resolved 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 \~1 s. In concert with techniques aimed at the interpretation of processes occurring in the fluid, novel seismic methods have emerged that are allowing the detection of stress changes in volcanic structures induced by magma movement. These methods include (1) ambient noise interferometry, in which the ambient seismic noise is used to probe temporal changes in volcanic structures; (2) the measurement of seismic anisotropy, where changes in the alignment of fluid-filled microcracks and pore space are monitored to assess the response of the crust to pressurization of a magmatic system; and (3) the detection of systematic changes in fault plane solutions of volcano-tectonic earthquakes caused by local stress perturbations during conduit pressurization. As new seismic methods refine our understanding of seismic sources and behavior of volcanic structures, we face new challenges in elucidating the physico-chemical processes that cause volcanic unrest and its seismic and gas-discharge manifestations. Future important goals toward meeting those challenges must include a better understanding of the key types of magma movement, degassing and boiling events that produce characteristic seismic phenomena, along with a quantitative understanding of multiphase fluid behavior under dynamic volcanic conditions. Realizing these goals will be essential for the development of an integrated model of volcanic behavior and will require multidisciplinary research involving detailed field measurements, laboratory experiments, and numerical modeling.