The chapter considers energy systems as an economic 'frontline' in a century-old struggle over whether economies should be organised around private or public forms of ownership. The ability to call on progressively larger quantities of energy – and to access energy in the high-quality forms of liquid fuels and electricity – has long been a measure of development. The relationship between economic growth and energy is complex. Economic landscapes are shaped by the ways in which energy is put to work in service of commerce, growth and the everyday need to make a living. The relationship between economic output and energy consumption can be described as the energy intensity of economic activity. Economic output is positively correlated with energy input at the national scale so that, as a general rule, countries with a higher level of GDP consume more energy than those with lower levels of economic development.