The AIST and the Nippon Sheet Glass Co., Ltd. have decided to develop a novel window glass for energy saving, through a joint research. The glass uses a semiconductor-to-metal phase transition material for automatic solar heat control, together with other materials for optical tailoring and multifunctional performance. The newly developed window glass is smart as to shade the extra solar radiation in summer but to introduce solar heat in winter, changing automatically in response to the environment temperature. Multifunction performances are achieved through precise material selection and structural optimization.