One of the fundamental objectives of an HVAC (heating, ventilation, air-conditioning) system is to create comfortable environments for occupants. The rule of thumb in building operation is the more energy a building consumes, the more comfortable it becomes. Saving energy and achieving comfort seem to conflict with each other. This might be true. However, are there opportunities to achieve both desires? In this talk, I will present a few case studies which demonstrate how we might both achieve comfort and save energy by using sensor networks in buildings. I will first report the latest thermal comfort survey results from around 150 commercial buildings in America. Second, I will use a recent field study in a pre-cooling office building to explain the cost of thermal discomfort in buildings. I will then present two examples that illustrate how more sensors might help adopting or designing new air conditioning systems. Finally, I will briefly go over my future research interests.