Energy-saving buildings have drawn increasing interest worldwide in the past 30 years, during which the growing population and expanding urbanization significantly increased the energy intensity of numerous cities. In the modern energy-saving buildings, smart windows are playing an important role in the efficient utilization of daylight and the intelligent control of heat exchange between indoor and outdoor, eventually reducing the energy waste associated with lighting and air-conditioning. The “intelligence” of smart windows originates from the responsive materials of which the optical properties are adaptive to temperature or applied voltage. Recently, the development of smart windows has been greatly motivated by the burgeoning nanomaterials. This chapter focuses on the development of heat and electro-responsive nanomaterials-based smart windows which outperform the conventional ones and, more importantly, likely to cost less for commercialization.