This study used existing digital technologies as probes to explore teenager’s use of exercise-related technology, their usage contexts, and their ideas for the design of ubiquitous technology that would persuade their peers to be more active in their daily lives. The teenagers formed two groups using two different types of step counters as their data collection device. Both groups used the same social website as their data logging tool and for social interaction. The one-week base-line plus six-weeks longitudinal study incorporated innovation workshops for generating user-centred design ideas: analysis of the findings drew out important lessons for the design of future devices. Key among these were: the impact of authority figures in restricting teenagers’ use of technologies, teenagers’ openness to sharing (privacy is not a concern), that data collection technologies must be ubiquitous but invisible, social interaction via technology is expected and positive messages reinforcing attainments of goals are valued (negative feedback is seen as demotivating).