Mankind has lived on the earth for hundreds of thousands of years in relative harmony with their natural surroundings. The earth and its climate were unaffected by the activities of early man. Within the last two hundred years, however, this peaceful co-existence has drastically changed as a result of our scientific knowledge and its widespread technological application. New agricultural techniques have greatly increased the productivity of the land and enabled the population to rise rapidly. The industrial revolution of the nineteenth century greatly increased the living standards in many countries, but at the same time it has polluted the earth to an unprecedented degree. This pollution is changing the face of the earth and its climate at an unforeseeable rate. If it is not checked our whole civilization is in peril. At the basis of these changes is the demand for more and more energy to drive our industries, to heat our homes and to power our transport and communications. All known ways of generating this energy affect the earth in one way or another, by using up the energy stored over geological timescales as coal or oil and by the pollution they cause. These sources of energy will ultimately be exhausted, but if we continue to rely on them, we may well cause irreversible climate change. It is therefore a matter of urgency to find safe and clean ways of generating energy. At the same time, it is necessary to reduce and if possible, eliminate all the other sources of pollution.