Measured energy use of buildings demonstrates large discrepancies even between buildings with the same function and located in similar climates. Among various factors contributing to the discrepancies, occupant behavior is found to be a key factor. How occupants set the comfort criteria, interact with building components and systems, and respond to environmental discomfort directly affects the operation of buildings and thus their energy use. On the other hand, it is also important to find out what sort of design methods can reduce building consumption in new and existing buildings. The primary focus of this seminar will be to provide an overview of occupant behavior and design efficiency in commercial office buildings.