Most modern high-rise buildings' facades use glasses for esthetics, daylight, and better environmental view through them. However, with the increasing use of a larger area of transparent glasses as walls, the visual privacy preservation of the indoors and heat energy loss through the windows are becoming a rising concern. Recent studies showed that nearly half of the energy consumed in a building goes to heating and air conditioning while approximately 40% of this energy is lost through windows. Windows with tunable optical properties that are generically termed "tunable windows or smart windows or switchable glass"are perceived as a potential solution for these problems. An optically tunable window can adjust the amount of daylighting passing through it, control the heat radiation, and/or change the transparency of the glasses for visual privacy preservation of indoors. Electrochromic glasses, polymer dispersed liquid crystal glasses, and suspended particle devices are available as commercial tunable windows but their high cost, limited optical performance, reliability, and operational complexity are hindering the widespread adaptation. Therefore, several other technologies for low-cost actively tunable windows capable of actively adjusting transparency are increasingly explored. However, most of such new technologies, working based on various optical principles, do not fulfill all the requirements of tunable windows. For instance, some can tune optical transmittance but do not affect energy transmission, and some can adjust heat radiation transmission but has a limited change in visual appearances. To fully take the advantage of the strengths as well as recognize the limitations of such emerging technologies, their optical principles need to be understood in-depth. Here, we review the recent developments in transmittance tunable windows by categorizing them based on the optics involved, namely, light absorption, reflection, and scattering. This in-depth review comprehensively discusses how the tunable window technologies compare to each other and offers insight into how their performance can be improved in the future.