Sedentary behavior in the workplace may be associated with negative health consequences. However, the effectiveness of interventions to reduce sedentary behavior is currently unclear. The accurate measurement of sedentary behavior is difficult, because the mechanisms that lead to the harmful health effects associated with sedentary behavior have not been well defined yet. A prerequisite to evaluate sedentary behavior is to classify sitting, standing and walking correctly. Currently, there is a lack of validation studies of activity monitors particularly in occupational settings. This study is designed to evaluate two different activity monitors (ActiGraph and activPAL) in the measurement of posture and motion during a broad range of office activities and specifically to test the performance limits of these devices in this special setting.