Smart windows admit selective transmission of sunlight for different kinds of indoor applications. They can be operated in the presence of light, current and heat. The combination of different layers in smart windows admits to filter or transfer the incoming light for our convenience. In recent years, use of smart windows for different sectors has been investigated through various device architectures. Especially, coupling such devices to fabricate an energy-efficient smart window has generated enormous interest for their application towards smart buildings. By selectively choosing materials, it is possible to tune the wavelength of the incoming light and so the purpose of smart windows can be altered manually. Various categories such as electrochromic, thermochromic, polymer dispersed liquid crystal films and suspended particle devices are showing interesting outcome in the presence of external factors which assure their potential use for different household and industrial purposes. In this chapter, the basic principle and performance of energy-efficient smart windows fabricated by different materials and their applications are discussed. The key findings related with each category have also been outlined.