Volcanoes are some of the most dramatic expressions of the powerful tectonic forces at work in the Earth beneath our feet. But volcanism, a profoundly important feature of Earth, and indeed of other planets and moons too, encompasses much more than just volcanoes themselves. On a planetary scale, volcanism is an indispensable heat release mechanism, which on Earth allows the conditions for life. IIt releases gases into the atmosphere and produces enormous volumes of rock, and spectacular landscapes - landscapes which, during major eruptions, can be completely reshaped in a matter of hours. Through geological time volcanism has shaped both climate and biological evolution, and volcanoes can affect human life, too, for both good and ill. Yet, even after much study, some of the fundamental aspects of volcanicity remain mysterious. This Very Short Introduction takes the readers into the inferno of a racing pyroclastic current, and the heart of a moving lava flow, as understood through the