Much research on smart environments has focused on homes and offices. With increasing physical inactivity and its health effects, augmenting gyms to motivate, assist and coach users to track and achieve their fitness goals is important. In this paper, we propose using smart mirrors to automatically detect and count repetitions of exercises, and to provide coaching, motivation and information to users. We present the concept and use cases, developed with 7 users (novices and fitness fanatics) through contextual inquiry, and discuss how the concept can be realized with current technologies.