We review recent advances in quantification of the sources of volcano-seismic signals. Since the 1990s, there have been remarkable advances in quantitative understanding of the sources of volcano-seismic signals. These were made by integrated contributions from observation, analysis, and modeling studies in volcano seismology. We focus on the following topics in this review paper: Broadband seismic observations on active volcanoes, waveform inversions of volcano-seismic signals, and source models based on resonances of a fluid-filled crack, bubble dynamics, and repeated fracture and healing of magma. Broadband seismic observations revealed the general existence of very-long-period seismic signals with oscillation periods longer than a few seconds beneath active volcanoes. Development of waveform inversion methods enabled us to estimate source mechanisms of volcano-seismic signals, which indicate that crack geometry is the most common in volcano-seismic sources. Source models have been successfully used to interpret dynamic interactions between volcanic fluids and the surrounding solid rock. Quantitative studies of volcano seismology are now reaching a mature stage, and links with geological and geochemical studies will be important future directions to achieve a better understanding of magmatic and hydrothermal processes beneath active volcanoes.