Hydrothermal alteration describes a process that progressively, and additively, modifies the chemical and physical properties of rock by fluid-rock interactions. At active volcanoes, mixtures of magmatic and meteoric fluids circulate within the rocks forming the volcano and, as a result, hydrothermal alteration can be pervasive. Because the properties of volcanic rocks, such as their strength or their permeability, play a role in dictating the hazard potential of a volcano, then it follows that hydrothermal alteration progressively modifies the hazard potential of a volcano. However, not only does subsurface hydrothermal alteration proceed largely imperceptibly, leading to unpredictable hydrothermal explosions and mass wasting events, but we also do not fully understand the timescales required for hydrothermal alteration, nor its influence of rock properties. As a result, and despite its potential importance, hydrothermal alteration is not routinely monitored at active volcanoes, and often does not feature in routine volcanic hazard assessments. In this seminar, I will outline recent, multidisciplinary advancements in our understanding of hydrothermal alteration, and its influence on volcanic hazards. I will also outline plans for future work on this topic, in the framework of the recently-funded ERC SYNERGY grant "ROTTnROCK".