Volcanic eruptions can expel many cubic kilometres of rock. Typically, a few per cent of the total mass of erupted materials is made up of gases. The rocks are just rocks – essentially inert bodies of lava or accumulations of tephra. The gases, on the other hand, become part of the atmosphere. Even non-erupting volcanoes can emit significant quantities of gas from fumarole vents. Volcanic gas emissions are by no means inert, either chemically or in their direct and indirect effects on the Earth's heat budget. Indeed, the global-scale climatic and environmental impacts of large eruptions arise principally through the action of this minor component of magmas – gas – and one chemical species in particular, sulphur. This chapter reviews the chemical and physical processes by which volcanic clouds affect the Earth's atmosphere and climate, emphasising the potential widespread and diverse knock-on impacts of major eruptions mediated through global climatic change. In many cases in the past, afflicted communities will almost certainly have had no idea that a volcanic eruption was the root cause of environmental stress.