Hydrothermal activity is common at active volcanoes. Volcanic gasses rise and form strong acids that lead to fluid-rock interactions affecting a rock’s mineral assemblage by dissolution and remineralization, eventually influencing essential rock parameters like strength and permeability. Despite the far-reaching consequences for the stability of a volcanic edifice, our understanding of extent and variability of hydrothermal alteration is often limited. Within Multi-Marex we aim to better understand the causes and effects of hydrothermal alteration and volcano stability on land and underwater. By close-range remote sensing, we analyze hydrothermal alteration, aiming to describe the morphology (shapes) and optical appearance of hydrothermally active sites over scales and to reveal the general pattern of alteration and its regional variability. We give an overview of optical methods for tracing hydrothermal alteration, compare patterns observed at different systems, and expand our view to the submarine regime. In particular, we compare the alteration pattern at Nisyros, a hydrothermally active volcano in the Aegean Arc with hydrothermal-dominated or magmatic-hydrothermal systems at locations elsewhere. The approaches and the pattern of hydrothermal alterations observed vary, but all systems have in common that there are patterns that can be detected and that indicate variability of gas flux and alteration and therefore zones of contrast considering materials, permeabilities, strength, or other physicochemical properties. Revealing these patterns is beneficial for detailed and focused further investigations and may be particularly useful for monitoring and future risk assessment studies.