In his Presidential Address to the British Association, Sir Harold Hartley traced the history of man's development and use of energy. The latter portions of this lecture, dealing with present-day problems of conserving existing resources and tapping new sources, solar and nuclear, are presented here. Sir Harold was Chairman of the World Power Conference which met in London last summer, and was the British government delegate to the United Nations Conference on Conservation and Utilization of Natural Resources held at Lake Success in 1949.