The usage of building energy accounts for 30-40% of total energy consumption in developed countries, exceeding the amount for industry or transportation. Around 50% energy for building services is contributed by heating, ventilation, and air-conditioning (HVAC) systems. More importantly, both building and HVAC energy consumptions are predicted to increase in the next two decades. Windows are considered as the least energy-efficient components of buildings. Therefore, smart windows are becoming increasingly important as they are capable of reducing HVAC energy usage by tuning the transmitted sunlight in a smart and favoured way: blocking solar irradiation on hot days, while letting it pass through on cold days. Compared with other type of smart windows, thermochromic windows have the unique advantages of cost-effectiveness, rational stimulus, and passive response. This book covers fabrication of vanadium dioxide-based smart windows, discusses various strategies to enhance their performance, and shares perspectives from the top scientists in this particular field.