Hourly energy use in commercial buildings shows periodic variation in daily and annual cycles. Moreover, the pattern of variation is such that Fourier series functional forms provide one of the best approaches for modeling this behavior. Results from numerous case studies have demonstrated the power of Fourier series as a modeling tool to represent hourly energy use in commercial buildings. This paper describes the Fourier series approaches that have been developed for modeling energy use in commercial buildings and presents the results of application to data collected from many sites.