The solar energy is presently the most promising as an alternative energy source, and R&D activities on its utilization are reviewed to provide a stimulant for similar efforts in energy-importing Korea. The so-called low-temperature utilization such as solar space heating and cooling, greenhouse, solar still and solar dehydration is more emphasized as practical means of implementation in developing countries. Whereas sophisticated solar thermal power generation employing solar concentration principle, heliochemical and helioelectric researches are briefly reviewed. Part I discusses the systematic classification of solar energy utilization, flat-plate solar collector upon which solar energy economics depends mostly, and the simplest system made of this collector, namely, solar water heater.