Long-period (LP) seismicity at active volcanoes reflects pressure fluctuations in the subsurface plumbing system, indicating internal dynamics and potentially signaling impending eruptions. LP events, characterized by high-frequency onsets and harmonic waveforms, are linked to fluid processes, including magma and hydrothermal fluid movement. They differ from volcano-tectonic (VT) earthquakes, which result from solid rock failures. LP events are often precursors to eruptions, with examples including the 1991 Pinatubo eruption. LP activity is more localized and associated with magma conduit interactions, while VT events are broader and linked to stress concentrations. LP events can be distinguished from VT events through spectral analysis, as they reflect source mechanisms rather than path effects. LP events are valuable for eruption forecasting due to their sensitivity to fluid dynamics and pressure changes. They are generated by fluid-driven processes, such as unsteady mass transport and thermodynamics, and can be modeled as resonant excitation in fluid-filled cracks. LP events and tremor share similar characteristics, suggesting a common source process. The study of LP events provides insights into magma and hydrothermal systems, with applications in understanding volcanic behavior and hazards. Future research aims to improve models of LP excitation mechanisms and use broadband instruments to monitor fluid movements before eruptions. LP activity is crucial for forecasting eruptions, particularly in stratovolcanoes, and requires further investigation into fluid dynamics and source modeling.Long-period (LP) seismicity at active volcanoes reflects pressure fluctuations in the subsurface plumbing system, indicating internal dynamics and potentially signaling impending eruptions. LP events, characterized by high-frequency onsets and harmonic waveforms, are linked to fluid processes, including magma and hydrothermal fluid movement. They differ from volcano-tectonic (VT) earthquakes, which result from solid rock failures. LP events are often precursors to eruptions, with examples including the 1991 Pinatubo eruption. LP activity is more localized and associated with magma conduit interactions, while VT events are broader and linked to stress concentrations. LP events can be distinguished from VT events through spectral analysis, as they reflect source mechanisms rather than path effects. LP events are valuable for eruption forecasting due to their sensitivity to fluid dynamics and pressure changes. They are generated by fluid-driven processes, such as unsteady mass transport and thermodynamics, and can be modeled as resonant excitation in fluid-filled cracks. LP events and tremor share similar characteristics, suggesting a common source process. The study of LP events provides insights into magma and hydrothermal systems, with applications in understanding volcanic behavior and hazards. Future research aims to improve models of LP excitation mechanisms and use broadband instruments to monitor fluid movements before eruptions. LP activity is crucial for forecasting eruptions, particularly in stratovolcanoes, and requires further investigation into fluid dynamics and source modeling.