Monitoring volcanoes is not limited to forecasting the onset of an eruption, but also includes confirmation of the occurrence of volcanic activity. In this chapter we will cover how infrasound, that is sound below human hearing, can be used as a monitoring tool. It is important to note that infrasound records atmospheric pressure perturbations and, as such, it requires coupling between its source and the atmosphere. Explosions are intuitive to understand; they produce audible acoustic energy, but also waves at lower frequencies below the audible range. This infrasound can be recorded instrumentally. Beyond explosion there is a broad range of volcanic activity that can be identified, located, tracked, or characterised using acoustic records. Since infrasound attenuation is relatively low, compared to seismic waves, it can be recorded at distances ranging from few kilometres (local) to thousands of kilometres away from the source (global). This chapter will describe how infrasound is used to monitor volcanic activity.