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Radio waves can benefit the welfare of humanity through other purposes than communications. Microwave power transmission (MPT) is one of the new technological frontiers. Solar power satellites (SPS) will provide a clean and limitless energy resource from space through this technique. This article answers the fundamental question of why we need to develop SPS from the viewpoint of critical global issues for mankind. It then reviews SPS research, mainly in Japan, in the 1980s and 1990s. These efforts include two MPT rocket experiments; a microwave-driven airplane experiment; and MPT technology developments, concepts, and systems studies of SPS. A summary of recent research committee activities on SPS and a road map of future SPS research plans in Japan concludes the article.