Energy is essential to life and economic development. Between 1900 and 2000 the energy consumption of humanity has been multiplied by 10 and the world's population by 3.5. With a piece of 10 cents of € we can buy in France about 1 kWh of electricity, the amount of energy supplied by two hard workers during a whole day. If the standard of living has increased so much over a century it is because we can get cheap and abundant energy.