Around 10 per cent of the world’s population (~800 million people) live within 100 km of a potentially active volcano. As volcanic areas become ever more populated and urbanised, the potential for volcanic disasters increases. Historically, human and economic losses from volcanic eruptions have been smaller than those from other natural hazards. However, many volcanoes are capable of eruptions much larger than those witnessed in historic times. Very large volcanic eruptions can threaten the global environment and civilisation. This chapter provides an overview of volcanoes and volcanic eruptions, with a particular focus on the hazards and risks they pose for society.