Prolonged sitting is a potential health risk, not only for people with an inactive lifestyle, but also for those who do meet the recommended amount of physical activity. In this paper, we present two waysto promote the reduction of sedentary behavior. First, we report onan experiment in which office workers (n = 40) received timely persuasive messages on their smartphones, advising them to take an active break whenever 30 minutes of almost uninterrupted computer activity was recorded. The messages resulted in a significant decrease in computer activity and a peak in physical activity, indicating that participants complied to the given advice and took short breaks upon receiving a message. Second, we developed SitCoach, a mobile application to nudge office workers from their seats. SitCoach monitors physical activity and sedentary behavior to provide timely feedback by means of suggested sitting breaks. The results of a user test showedthat the general awareness of the importance of sitting breaks is low. In addition, the ability to take sitting breaks was considered to be highly dependent on external factors. This suggests that raising awareness and increasing self-efficacy are important prerequisitesfor a successful intervention to reduce sedentary behavior.