Since its beginning, life has relied upon the sun to sustain a good climate on Earth. Now, the use of solar energy is starting to be viewed as a great, non-polluting means of energy. Solar energy is already being used as a source of heat. The principle that a black surface exposed to the sun will absorb solar energy is the basis of several million domestic hot water heaters used in a dozen countries, including Japan, Australia and Israel. A more advanced system could be applied to home heating and cooling. If solar energy someday replaces oil and coal, we will enjoy more power, cleaner air, and better health.